diff --git a/src/adapters/ExplicitModelAdapter.h b/src/adapters/ExplicitModelAdapter.h
index cc2cfa6db..a6e517038 100644
--- a/src/adapters/ExplicitModelAdapter.h
+++ b/src/adapters/ExplicitModelAdapter.h
@@ -233,7 +233,7 @@ namespace storm {
              * @param actionIndex The index of the action label to select.
              * @return A list of lists of active commands or nothing.
              */
-            static boost::optional<std::vector<std::list<std::reference_wrapper<storm::prism::Command const>>>> getActiveCommandsByAction(storm::prism::Program const& program, StateType const* state, uint_fast64_t const& actionIndex) {
+            static boost::optional<std::vector<std::list<std::reference_wrapper<storm::prism::Command const>>>> getActiveCommandsByActionIndex(storm::prism::Program const& program, StateType const* state, uint_fast64_t const& actionIndex) {
                 boost::optional<std::vector<std::list<std::reference_wrapper<storm::prism::Command const>>>> result((std::vector<std::list<std::reference_wrapper<storm::prism::Command const>>>()));
                 
                 // Iterate over all modules.
@@ -288,14 +288,14 @@ namespace storm {
                         storm::prism::Command const& command = module.getCommand(j);
                         
                         // Only consider unlabeled commands.
-                        if (!command.isLabeled()) continue;
+                        if (command.isLabeled()) continue;
 
                         // Skip the command, if it is not enabled.
                         if (!command.getGuardExpression().evaluateAsBool(currentState)) {
                             continue;
                         }
                         
-                        result.push_back(Choice<ValueType>(""));
+                        result.push_back(Choice<ValueType>());
                         Choice<ValueType>& choice = result.back();
                         choice.addChoiceLabel(command.getGlobalIndex());
                         
@@ -331,9 +331,9 @@ namespace storm {
             static std::list<Choice<ValueType>> getLabeledTransitions(storm::prism::Program const& program, StateInformation& stateInformation, VariableInformation const& variableInformation, uint_fast64_t stateIndex, std::queue<uint_fast64_t>& stateQueue) {
                 std::list<Choice<ValueType>> result;
                 
-                for (std::string const& action : program.getActions()) {
+                for (uint_fast64_t actionIndex : program.getActionIndices()) {
                     StateType const* currentState = stateInformation.reachableStates[stateIndex];
-                    boost::optional<std::vector<std::list<std::reference_wrapper<storm::prism::Command const>>>> optionalActiveCommandLists = getActiveCommandsByAction(program, currentState, action);
+                    boost::optional<std::vector<std::list<std::reference_wrapper<storm::prism::Command const>>>> optionalActiveCommandLists = getActiveCommandsByActionIndex(program, currentState, actionIndex);
                     
                     // Only process this action label, if there is at least one feasible solution.
                     if (optionalActiveCommandLists) {
@@ -401,7 +401,7 @@ namespace storm {
                             // At this point, we applied all commands of the current command combination and newTargetStates
                             // contains all target states and their respective probabilities. That means we are now ready to
                             // add the choice to the list of transitions.
-                            result.push_back(Choice<ValueType>(action));
+                            result.push_back(Choice<ValueType>(actionIndex));
                             
                             // Now create the actual distribution.
                             Choice<ValueType>& choice = result.back();
@@ -526,7 +526,7 @@ namespace storm {
                         // Then, based on whether the model is deterministic or not, either add the choices individually
                         // or compose them to one choice.
                         if (deterministicModel) {
-                            Choice<ValueType> globalChoice("");
+                            Choice<ValueType> globalChoice;
                             std::map<uint_fast64_t, ValueType> stateToRewardMap;
                             boost::container::flat_set<uint_fast64_t> allChoiceLabels;
                             
@@ -538,7 +538,7 @@ namespace storm {
                                     
                                     // Now add all rewards that match this choice.
                                     for (auto const& transitionReward : transitionRewards) {
-                                        if (!transitionReward.empty() && transitionReward.getStatePredicateExpression().evaluateAsBool(stateInformation.reachableStates.at(currentState))) {
+                                        if (!transitionReward.isLabeled() && transitionReward.getStatePredicateExpression().evaluateAsBool(stateInformation.reachableStates.at(currentState))) {
                                             stateToRewardMap[stateProbabilityPair.first] += ValueType(transitionReward.getRewardValueExpression().evaluateAsDouble(stateInformation.reachableStates.at(currentState)));
                                         }
                                     }
diff --git a/src/parser/PrismParser.cpp b/src/parser/PrismParser.cpp
index e1a0791cb..66e239c3a 100644
--- a/src/parser/PrismParser.cpp
+++ b/src/parser/PrismParser.cpp
@@ -111,12 +111,12 @@ namespace storm {
             stateRewardDefinition = (expressionParser > qi::lit(":") > expressionParser >> qi::lit(";"))[qi::_val = phoenix::bind(&PrismParser::createStateReward, phoenix::ref(*this), qi::_1, qi::_2)];
             stateRewardDefinition.name("state reward definition");
             
-            transitionRewardDefinition = (qi::lit("[") > -(identifier[qi::_a = qi::_1]) > qi::lit("]") > expressionParser > qi::lit(":") > expressionParser > qi::lit(";"))[qi::_val = phoenix::bind(&PrismParser::createTransitionReward, phoenix::ref(*this), qi::_a, qi::_2, qi::_3)];
+            transitionRewardDefinition = (qi::lit("[") > -(identifier[qi::_a = qi::_1]) > qi::lit("]") > expressionParser > qi::lit(":") > expressionParser > qi::lit(";"))[qi::_val = phoenix::bind(&PrismParser::createTransitionReward, phoenix::ref(*this), qi::_a, qi::_2, qi::_3, qi::_r1)];
             transitionRewardDefinition.name("transition reward definition");
             
             rewardModelDefinition = (qi::lit("rewards") > -(qi::lit("\"") > identifier[qi::_a = qi::_1] > qi::lit("\""))
                                      > +(   stateRewardDefinition[phoenix::push_back(qi::_b, qi::_1)]
-                                         |   transitionRewardDefinition[phoenix::push_back(qi::_c, qi::_1)]
+                                         |  transitionRewardDefinition(qi::_r1)[phoenix::push_back(qi::_c, qi::_1)]
                                          )
                                      >> qi::lit("endrewards"))[qi::_val = phoenix::bind(&PrismParser::createRewardModel, phoenix::ref(*this), qi::_a, qi::_b, qi::_c)];
             rewardModelDefinition.name("reward model definition");
@@ -161,7 +161,7 @@ namespace storm {
                          | globalVariableDefinition(qi::_a)
                          | (moduleRenaming(qi::_a) | moduleDefinition(qi::_a))[phoenix::push_back(phoenix::bind(&GlobalProgramInformation::modules, qi::_a), qi::_1)]
                          | initialStatesConstruct(qi::_a)
-                         | rewardModelDefinition[phoenix::push_back(phoenix::bind(&GlobalProgramInformation::rewardModels, qi::_a), qi::_1)] 
+                         | rewardModelDefinition(qi::_a)[phoenix::push_back(phoenix::bind(&GlobalProgramInformation::rewardModels, qi::_a), qi::_1)]
                          | labelDefinition[phoenix::push_back(phoenix::bind(&GlobalProgramInformation::labels, qi::_a), qi::_1)] 
                          | formulaDefinition[phoenix::push_back(phoenix::bind(&GlobalProgramInformation::formulas, qi::_a), qi::_1)]
                      )
@@ -340,8 +340,10 @@ namespace storm {
             return storm::prism::StateReward(statePredicateExpression, rewardValueExpression, this->getFilename());
         }
         
-        storm::prism::TransitionReward PrismParser::createTransitionReward(std::string const& actionName, storm::expressions::Expression statePredicateExpression, storm::expressions::Expression rewardValueExpression) const {
-            return storm::prism::TransitionReward(actionName, statePredicateExpression, rewardValueExpression, this->getFilename());
+        storm::prism::TransitionReward PrismParser::createTransitionReward(std::string const& actionName, storm::expressions::Expression statePredicateExpression, storm::expressions::Expression rewardValueExpression, GlobalProgramInformation& globalProgramInformation) const {
+            auto const& nameIndexPair = globalProgramInformation.actionIndices.find(actionName);
+            STORM_LOG_THROW(nameIndexPair != globalProgramInformation.actionIndices.end(), storm::exceptions::WrongFormatException, "Transition reward refers to illegal action '" << actionName << "'.");
+            return storm::prism::TransitionReward(nameIndexPair->second, actionName, statePredicateExpression, rewardValueExpression, this->getFilename());
         }
         
         storm::prism::Assignment PrismParser::createAssignment(std::string const& variableName, storm::expressions::Expression assignedExpression) const {
@@ -502,7 +504,7 @@ namespace storm {
         }
         
         storm::prism::Program PrismParser::createProgram(GlobalProgramInformation const& globalProgramInformation) const {
-            return storm::prism::Program(manager, globalProgramInformation.modelType, globalProgramInformation.constants, globalProgramInformation.globalBooleanVariables, globalProgramInformation.globalIntegerVariables, globalProgramInformation.formulas, globalProgramInformation.modules, globalProgramInformation.rewardModels, this->secondRun && !globalProgramInformation.hasInitialConstruct, globalProgramInformation.initialConstruct, globalProgramInformation.labels, this->getFilename(), 1, this->secondRun);
+            return storm::prism::Program(manager, globalProgramInformation.modelType, globalProgramInformation.constants, globalProgramInformation.globalBooleanVariables, globalProgramInformation.globalIntegerVariables, globalProgramInformation.formulas, globalProgramInformation.modules, globalProgramInformation.actionIndices, globalProgramInformation.rewardModels, this->secondRun && !globalProgramInformation.hasInitialConstruct, globalProgramInformation.initialConstruct, globalProgramInformation.labels, this->getFilename(), 1, this->secondRun);
         }
     } // namespace parser
 } // namespace storm
diff --git a/src/parser/PrismParser.h b/src/parser/PrismParser.h
index 9272d1d0e..539c9bc48 100644
--- a/src/parser/PrismParser.h
+++ b/src/parser/PrismParser.h
@@ -193,9 +193,9 @@ namespace storm {
             qi::rule<Iterator, storm::prism::Assignment(), Skipper> assignmentDefinition;
             
             // Rules for reward definitions.
-            qi::rule<Iterator, storm::prism::RewardModel(), qi::locals<std::string, std::vector<storm::prism::StateReward>, std::vector<storm::prism::TransitionReward>>, Skipper> rewardModelDefinition;
+            qi::rule<Iterator, storm::prism::RewardModel(GlobalProgramInformation&), qi::locals<std::string, std::vector<storm::prism::StateReward>, std::vector<storm::prism::TransitionReward>>, Skipper> rewardModelDefinition;
             qi::rule<Iterator, storm::prism::StateReward(), Skipper> stateRewardDefinition;
-            qi::rule<Iterator, storm::prism::TransitionReward(), qi::locals<std::string>, Skipper> transitionRewardDefinition;
+            qi::rule<Iterator, storm::prism::TransitionReward(GlobalProgramInformation&), qi::locals<std::string>, Skipper> transitionRewardDefinition;
             
             // Rules for initial states expression.
             qi::rule<Iterator, qi::unused_type(GlobalProgramInformation&), Skipper> initialStatesConstruct;
@@ -232,7 +232,7 @@ namespace storm {
             storm::prism::Label createLabel(std::string const& labelName, storm::expressions::Expression expression) const;
             storm::prism::RewardModel createRewardModel(std::string const& rewardModelName, std::vector<storm::prism::StateReward> const& stateRewards, std::vector<storm::prism::TransitionReward> const& transitionRewards) const;
             storm::prism::StateReward createStateReward(storm::expressions::Expression statePredicateExpression, storm::expressions::Expression rewardValueExpression) const;
-            storm::prism::TransitionReward createTransitionReward(std::string const& actionName, storm::expressions::Expression statePredicateExpression, storm::expressions::Expression rewardValueExpression) const;
+            storm::prism::TransitionReward createTransitionReward(std::string const& actionName, storm::expressions::Expression statePredicateExpression, storm::expressions::Expression rewardValueExpression, GlobalProgramInformation& globalProgramInformation) const;
             storm::prism::Assignment createAssignment(std::string const& variableName, storm::expressions::Expression assignedExpression) const;
             storm::prism::Update createUpdate(storm::expressions::Expression likelihoodExpression, std::vector<storm::prism::Assignment> const& assignments, GlobalProgramInformation& globalProgramInformation) const;
             storm::prism::Command createCommand(std::string const& actionName, storm::expressions::Expression guardExpression, std::vector<storm::prism::Update> const& updates, GlobalProgramInformation& globalProgramInformation) const;
diff --git a/src/storage/prism/Module.cpp b/src/storage/prism/Module.cpp
index 3259c2673..bc40cbd78 100644
--- a/src/storage/prism/Module.cpp
+++ b/src/storage/prism/Module.cpp
@@ -10,11 +10,11 @@ namespace storm {
             // Intentionally left empty.
         }
         
-        Module::Module(std::string const& moduleName, std::vector<storm::prism::BooleanVariable> const& booleanVariables, std::vector<storm::prism::IntegerVariable> const& integerVariables, std::vector<storm::prism::Command> const& commands, std::string const& renamedFromModule, std::map<std::string, std::string> const& renaming, std::string const& filename, uint_fast64_t lineNumber) : LocatedInformation(filename, lineNumber), moduleName(moduleName), booleanVariables(booleanVariables), booleanVariableToIndexMap(), integerVariables(integerVariables), integerVariableToIndexMap(), commands(commands), actions(), actionIndices(), actionsToCommandIndexMap(), renamedFromModule(renamedFromModule), renaming(renaming) {
+        Module::Module(std::string const& moduleName, std::vector<storm::prism::BooleanVariable> const& booleanVariables, std::vector<storm::prism::IntegerVariable> const& integerVariables, std::vector<storm::prism::Command> const& commands, std::string const& renamedFromModule, std::map<std::string, std::string> const& renaming, std::string const& filename, uint_fast64_t lineNumber) : LocatedInformation(filename, lineNumber), moduleName(moduleName), booleanVariables(booleanVariables), booleanVariableToIndexMap(), integerVariables(integerVariables), integerVariableToIndexMap(), commands(commands), actionIndices(), actionIndicesToCommandIndexMap(), renamedFromModule(renamedFromModule), renaming(renaming) {
             // Initialize the internal mappings for fast information retrieval.
             this->createMappings();
         }
-                
+        
         std::size_t Module::getNumberOfBooleanVariables() const {
             return this->booleanVariables.size();
         }
@@ -32,7 +32,7 @@ namespace storm {
         std::vector<storm::prism::BooleanVariable> const& Module::getBooleanVariables() const {
             return this->booleanVariables;
         }
-
+        
         storm::prism::IntegerVariable const& Module::getIntegerVariable(std::string const& variableName) const {
             auto const& nameIndexPair = this->integerVariableToIndexMap.find(variableName);
             STORM_LOG_THROW(nameIndexPair != this->integerVariableToIndexMap.end(), storm::exceptions::InvalidArgumentException, "Unknown integer variable '" << variableName << "'.");
@@ -67,16 +67,11 @@ namespace storm {
             return this->moduleName;
         }
         
-        std::set<std::string> const& Module::getActions() const {
-            return this->actions;
+        std::set<uint_fast64_t> const& Module::getActionIndices() const {
+            return this->actionIndices;
         }
         
-        bool Module::hasAction(std::string const& action) const {
-            auto const& actionEntry = this->actions.find(action);
-            return actionEntry != this->actions.end();
-        }
-        
-        bool Module::hasActionIndex(uint_fast64_t const& actionIndex) const {
+        bool Module::hasActionIndex(uint_fast64_t actionIndex) const {
             return this->actionIndices.find(actionIndex) != this->actionIndices.end();
         }
         
@@ -94,15 +89,6 @@ namespace storm {
             return this->renaming;
         }
         
-        std::set<uint_fast64_t> const& Module::getCommandIndicesByAction(std::string const& action) const {
-            auto actionsCommandSetPair = this->actionsToCommandIndexMap.find(action);
-            if (actionsCommandSetPair != this->actionsToCommandIndexMap.end()) {
-                return actionsCommandSetPair->second;
-            }
-            
-            STORM_LOG_THROW(false, storm::exceptions::OutOfRangeException, "Action name '" << action << "' does not exist in module.");
-        }
-        
         std::set<uint_fast64_t> const& Module::getCommandIndicesByActionIndex(uint_fast64_t actionIndex) const {
             auto actionIndicesCommandSetPair = this->actionIndicesToCommandIndexMap.find(actionIndex);
             if (actionIndicesCommandSetPair != this->actionIndicesToCommandIndexMap.end()) {
@@ -114,7 +100,6 @@ namespace storm {
         
         void Module::createMappings() {
             // Clear the current mappings.
-            this->actionsToCommandIndexMap.clear();
             this->actionIndicesToCommandIndexMap.clear();
             this->booleanVariableToIndexMap.clear();
             this->integerVariableToIndexMap.clear();
@@ -129,30 +114,18 @@ namespace storm {
             
             // Add the mapping for all commands.
             for (uint_fast64_t i = 0; i < this->commands.size(); i++) {
-                std::string const& action = this->commands[i].getActionName();
-                uint_fast64_t actionIndex = this->commands[i].getActionIndex();
-                if (action != "") {
-                    if (this->actionsToCommandIndexMap.find(action) == this->actionsToCommandIndexMap.end()) {
-                        this->actionsToCommandIndexMap.emplace(action, std::set<uint_fast64_t>());
-                    }
+                if (this->commands[i].isLabeled()) {
+                    uint_fast64_t actionIndex = this->commands[i].getActionIndex();
                     if (this->actionIndicesToCommandIndexMap.find(actionIndex) == this->actionIndicesToCommandIndexMap.end()) {
                         this->actionIndicesToCommandIndexMap.emplace(actionIndex, std::set<uint_fast64_t>());
                     }
-                    this->actionsToCommandIndexMap[action].insert(i);
                     this->actionIndicesToCommandIndexMap[actionIndex].insert(i);
-                    
-                    this->actions.insert(action);
-                    this->actionIndices.insert(this->commands[i].getActionIndex());
+                    this->actionIndices.insert(actionIndex);
                 }
             }
             
             // For all actions that are "in the module", but for which no command exists, we add the mapping to an empty
             // set of commands.
-            for (auto const& action : this->actions) {
-                if (this->actionsToCommandIndexMap.find(action) == this->actionsToCommandIndexMap.end()) {
-                    this->actionsToCommandIndexMap[action] = std::set<uint_fast64_t>();
-                }
-            }
             for (auto const& actionIndex : this->actionIndices) {
                 if (this->actionIndicesToCommandIndexMap.find(actionIndex) == this->actionIndicesToCommandIndexMap.end()) {
                     this->actionIndicesToCommandIndexMap[actionIndex] = std::set<uint_fast64_t>();
diff --git a/src/storage/prism/Module.h b/src/storage/prism/Module.h
index c22437fdc..e8686cce5 100644
--- a/src/storage/prism/Module.h
+++ b/src/storage/prism/Module.h
@@ -135,27 +135,19 @@ namespace storm {
             std::string const& getName() const;
             
             /*!
-             * Retrieves the set of actions present in this module.
+             * Retrieves the set of action indices present in this module.
              *
-             * @return the set of actions present in this module.
+             * @return the set of action indices present in this module.
              */
-            std::set<std::string> const& getActions() const;
+            std::set<uint_fast64_t> const& getActionIndices() const;
             
-            /*!
-             * Retrieves whether or not this module contains a command labeled with the given action.
-             *
-             * @param action The action name to look for in this module.
-             * @return True iff the module has at least one command labeled with the given action.
-             */
-            bool hasAction(std::string const& action) const;
-
             /*!
              * Retrieves whether or not this module contains a command labeled with the given action index.
              *
-             * @param action The action name to look for in this module.
+             * @param actionIndex The index of the action to look for in this module.
              * @return True iff the module has at least one command labeled with the given action index.
              */
-            bool hasActionIndex(uint_fast64_t const& actionIndex) const;
+            bool hasActionIndex(uint_fast64_t actionIndex) const;
             
             /*!
              * Retrieves whether this module was created from another module via renaming.
@@ -180,14 +172,6 @@ namespace storm {
              */
             std::map<std::string, std::string> const& getRenaming() const;
             
-            /*!
-             * Retrieves the indices of all commands within this module that are labelled by the given action.
-             *
-             * @param action The action with which the commands have to be labelled.
-             * @return A set of indices of commands that are labelled with the given action.
-             */
-            std::set<uint_fast64_t> const& getCommandIndicesByAction(std::string const& action) const;
-            
             /*!
              * Retrieves the indices of all commands within this module that are labelled by the given action.
              *
@@ -238,15 +222,9 @@ namespace storm {
             // The commands associated with the module.
             std::vector<storm::prism::Command> commands;
             
-            // The set of actions present in this module.
-            std::set<std::string> actions;
-            
             // The set of action indices present in this module.
             std::set<uint_fast64_t> actionIndices;
             
-            // A map of actions to the set of commands labeled with this action.
-            std::map<std::string, std::set<uint_fast64_t>> actionsToCommandIndexMap;
-
             // A map of actions to the set of commands labeled with this action.
             std::map<uint_fast64_t, std::set<uint_fast64_t>> actionIndicesToCommandIndexMap;
             
diff --git a/src/storage/prism/Program.cpp b/src/storage/prism/Program.cpp
index 26bf980e8..013d3d77d 100644
--- a/src/storage/prism/Program.cpp
+++ b/src/storage/prism/Program.cpp
@@ -11,7 +11,7 @@
 
 namespace storm {
     namespace prism {
-        Program::Program(std::shared_ptr<storm::expressions::ExpressionManager> manager, ModelType modelType, std::vector<Constant> const& constants, std::vector<BooleanVariable> const& globalBooleanVariables, std::vector<IntegerVariable> const& globalIntegerVariables, std::vector<Formula> const& formulas, std::vector<Module> const& modules, std::vector<RewardModel> const& rewardModels, bool fixInitialConstruct, storm::prism::InitialConstruct const& initialConstruct, std::vector<Label> const& labels, std::string const& filename, uint_fast64_t lineNumber, bool checkValidity) : LocatedInformation(filename, lineNumber), manager(manager), modelType(modelType), constants(constants), constantToIndexMap(), globalBooleanVariables(globalBooleanVariables), globalBooleanVariableToIndexMap(), globalIntegerVariables(globalIntegerVariables), globalIntegerVariableToIndexMap(), formulas(formulas), formulaToIndexMap(), modules(modules), moduleToIndexMap(), rewardModels(rewardModels), rewardModelToIndexMap(), initialConstruct(initialConstruct), labels(labels), labelToIndexMap(), actions(), actionsToModuleIndexMap(), variableToModuleIndexMap() {
+        Program::Program(std::shared_ptr<storm::expressions::ExpressionManager> manager, ModelType modelType, std::vector<Constant> const& constants, std::vector<BooleanVariable> const& globalBooleanVariables, std::vector<IntegerVariable> const& globalIntegerVariables, std::vector<Formula> const& formulas, std::vector<Module> const& modules, std::map<std::string, uint_fast64_t> const& actionToIndexMap, std::vector<RewardModel> const& rewardModels, bool fixInitialConstruct, storm::prism::InitialConstruct const& initialConstruct, std::vector<Label> const& labels, std::string const& filename, uint_fast64_t lineNumber, bool checkValidity) : LocatedInformation(filename, lineNumber), manager(manager), modelType(modelType), constants(constants), constantToIndexMap(), globalBooleanVariables(globalBooleanVariables), globalBooleanVariableToIndexMap(), globalIntegerVariables(globalIntegerVariables), globalIntegerVariableToIndexMap(), formulas(formulas), formulaToIndexMap(), modules(modules), moduleToIndexMap(), rewardModels(rewardModels), rewardModelToIndexMap(), initialConstruct(initialConstruct), labels(labels), labelToIndexMap(), actionToIndexMap(actionToIndexMap), actions(), actionIndices(), actionIndicesToModuleIndexMap(), variableToModuleIndexMap() {
             this->createMappings();
             
             // Create a new initial construct if the corresponding flag was set.
@@ -124,6 +124,10 @@ namespace storm {
             return this->modules;
         }
         
+        std::map<std::string, uint_fast64_t> const& Program::getActionNameToIndexMapping() const {
+            return actionToIndexMap;
+        }
+        
         storm::prism::InitialConstruct const& Program::getInitialConstruct() const {
             return this->initialConstruct;
         }
@@ -132,9 +136,19 @@ namespace storm {
             return this->actions;
         }
         
+        std::set<uint_fast64_t> const& Program::getActionIndices() const {
+            return this->actionIndices;
+        }
+        
         std::set<uint_fast64_t> const& Program::getModuleIndicesByAction(std::string const& action) const {
-            auto const& actionModuleSetPair = this->actionsToModuleIndexMap.find(action);
-            STORM_LOG_THROW(actionModuleSetPair != this->actionsToModuleIndexMap.end(), storm::exceptions::OutOfRangeException, "Action name '" << action << "' does not exist.");
+            auto const& nameIndexPair = this->actionToIndexMap.find(action);
+            STORM_LOG_THROW(nameIndexPair != this->actionToIndexMap.end(), storm::exceptions::OutOfRangeException, "Action name '" << action << "' does not exist.");
+            return this->getModuleIndicesByActionIndex(nameIndexPair->second);
+        }
+        
+        std::set<uint_fast64_t> const& Program::getModuleIndicesByActionIndex(uint_fast64_t actionIndex) const {
+            auto const& actionModuleSetPair = this->actionIndicesToModuleIndexMap.find(actionIndex);
+            STORM_LOG_THROW(actionModuleSetPair != this->actionIndicesToModuleIndexMap.end(), storm::exceptions::OutOfRangeException, "Action name '" << actionIndex << "' does not exist.");
             return actionModuleSetPair->second;
         }
         
@@ -188,7 +202,7 @@ namespace storm {
                 newModules.push_back(module.restrictCommands(indexSet));
             }
             
-            return Program(this->manager, this->getModelType(), this->getConstants(), this->getGlobalBooleanVariables(), this->getGlobalIntegerVariables(), this->getFormulas(), newModules, this->getRewardModels(), false, this->getInitialConstruct(), this->getLabels());
+            return Program(this->manager, this->getModelType(), this->getConstants(), this->getGlobalBooleanVariables(), this->getGlobalIntegerVariables(), this->getFormulas(), newModules, this->getActionNameToIndexMapping(), this->getRewardModels(), false, this->getInitialConstruct(), this->getLabels());
         }
         
         void Program::createMappings() {
@@ -215,17 +229,21 @@ namespace storm {
                 this->labelToIndexMap[this->getLabels()[labelIndex].getName()] = labelIndex;
             }
             
+            for (auto const& actionIndexPair : this->getActionNameToIndexMapping()) {
+                this->actions.insert(actionIndexPair.first);
+                this->actionIndices.insert(actionIndexPair.second);
+            }
+            
             // Build the mapping from action names to module indices so that the lookup can later be performed quickly.
             for (unsigned int moduleIndex = 0; moduleIndex < this->getNumberOfModules(); moduleIndex++) {
                 Module const& module = this->getModule(moduleIndex);
                 
-                for (auto const& action : module.getActions()) {
-                    auto const& actionModuleIndicesPair = this->actionsToModuleIndexMap.find(action);
-                    if (actionModuleIndicesPair == this->actionsToModuleIndexMap.end()) {
-                        this->actionsToModuleIndexMap[action] = std::set<uint_fast64_t>();
+                for (auto const& actionIndex : module.getActionIndices()) {
+                    auto const& actionModuleIndicesPair = this->actionIndicesToModuleIndexMap.find(actionIndex);
+                    if (actionModuleIndicesPair == this->actionIndicesToModuleIndexMap.end()) {
+                        this->actionIndicesToModuleIndexMap[actionIndex] = std::set<uint_fast64_t>();
                     }
-                    this->actionsToModuleIndexMap[action].insert(moduleIndex);
-                    this->actions.insert(action);
+                    this->actionIndicesToModuleIndexMap[actionIndex].insert(moduleIndex);
                 }
                 
                 // Put in the appropriate entries for the mapping from variable names to module index.
@@ -279,7 +297,7 @@ namespace storm {
                 STORM_LOG_THROW(definedUndefinedConstants.find(constantExpressionPair.first) != definedUndefinedConstants.end(), storm::exceptions::InvalidArgumentException, "Unable to define non-existant constant.");
             }
             
-            return Program(this->manager, this->getModelType(), newConstants, this->getGlobalBooleanVariables(), this->getGlobalIntegerVariables(), this->getFormulas(), this->getModules(), this->getRewardModels(), false, this->getInitialConstruct(), this->getLabels());
+            return Program(this->manager, this->getModelType(), newConstants, this->getGlobalBooleanVariables(), this->getGlobalIntegerVariables(), this->getFormulas(), this->getModules(), this->getActionNameToIndexMapping(), this->getRewardModels(), false, this->getInitialConstruct(), this->getLabels());
         }
         
         Program Program::substituteConstants() const {
@@ -338,7 +356,7 @@ namespace storm {
                 newLabels.emplace_back(label.substitute(constantSubstitution));
             }
             
-            return Program(this->manager, this->getModelType(), newConstants, newBooleanVariables, newIntegerVariables, newFormulas, newModules, newRewardModels, false, newInitialConstruct, newLabels);
+            return Program(this->manager, this->getModelType(), newConstants, newBooleanVariables, newIntegerVariables, newFormulas, newModules, this->getActionNameToIndexMapping(), newRewardModels, false, newInitialConstruct, newLabels);
         }
         
         void Program::checkValidity() const {
diff --git a/src/storage/prism/Program.h b/src/storage/prism/Program.h
index 6758e0881..5e37f5095 100644
--- a/src/storage/prism/Program.h
+++ b/src/storage/prism/Program.h
@@ -35,6 +35,7 @@ namespace storm {
              * @param globalIntegerVariables The global integer variables of the program.
              * @param formulas The formulas defined in the program.
              * @param modules The modules of the program.
+             * @param actionToIndexMap A mapping of action names to their indices.
              * @param fixInitialConstruct A flag that indicates whether the given initial construct is to be ignored and
              * replaced by a new one created from the initial values of the variables.
              * @param initialConstruct The initial construct of the program. If the initial construct specifies "false"
@@ -46,7 +47,7 @@ namespace storm {
              * @param lineNumber The line number in which the program is defined.
              * @param checkValidity If set to true, the program is checked for validity.
              */
-            Program(std::shared_ptr<storm::expressions::ExpressionManager> manager, ModelType modelType, std::vector<Constant> const& constants, std::vector<BooleanVariable> const& globalBooleanVariables, std::vector<IntegerVariable> const& globalIntegerVariables, std::vector<Formula> const& formulas, std::vector<Module> const& modules, std::vector<RewardModel> const& rewardModels, bool fixInitialConstruct, storm::prism::InitialConstruct const& initialConstruct, std::vector<Label> const& labels, std::string const& filename = "", uint_fast64_t lineNumber = 0, bool checkValidity = true);
+            Program(std::shared_ptr<storm::expressions::ExpressionManager> manager, ModelType modelType, std::vector<Constant> const& constants, std::vector<BooleanVariable> const& globalBooleanVariables, std::vector<IntegerVariable> const& globalIntegerVariables, std::vector<Formula> const& formulas, std::vector<Module> const& modules, std::map<std::string, uint_fast64_t> const& actionToIndexMap, std::vector<RewardModel> const& rewardModels, bool fixInitialConstruct, storm::prism::InitialConstruct const& initialConstruct, std::vector<Label> const& labels, std::string const& filename = "", uint_fast64_t lineNumber = 0, bool checkValidity = true);
             
             // Provide default implementations for constructors and assignments.
             Program() = default;
@@ -189,6 +190,13 @@ namespace storm {
              */
             std::vector<Module> const& getModules() const;
             
+            /*!
+             * Retrieves the mapping of action names to their indices.
+             *
+             * @return The mapping of action names to their indices.
+             */
+            std::map<std::string, uint_fast64_t> const& getActionNameToIndexMapping() const;
+            
             /*!
              * Retrieves the initial construct of the program.
              *
@@ -203,6 +211,13 @@ namespace storm {
              */
             std::set<std::string> const& getActions() const;
             
+            /*!
+             * Retrieves the set of action indices present in the program.
+             *
+             * @return The set of action indices present in the program.
+             */
+            std::set<uint_fast64_t> const& getActionIndices() const;
+            
             /*!
              * Retrieves the indices of all modules within this program that contain commands that are labelled with the
              * given action.
@@ -211,6 +226,15 @@ namespace storm {
              * @return A set of indices of all matching modules.
              */
             std::set<uint_fast64_t> const& getModuleIndicesByAction(std::string const& action) const;
+
+            /*!
+             * Retrieves the indices of all modules within this program that contain commands that are labelled with the
+             * given action index.
+             *
+             * @param actionIndex The index of the action the modules are supposed to possess.
+             * @return A set of indices of all matching modules.
+             */
+            std::set<uint_fast64_t> const& getModuleIndicesByActionIndex(uint_fast64_t actionIndex) const;
             
             /*!
              * Retrieves the index of the module in which the given variable name was declared.
@@ -381,11 +405,17 @@ namespace storm {
             // A mapping from label names to their corresponding indices.
             std::map<std::string, uint_fast64_t> labelToIndexMap;
             
+            // A mapping from action names to their indices.
+            std::map<std::string, uint_fast64_t> actionToIndexMap;
+            
             // The set of actions present in this program.
             std::set<std::string> actions;
+
+            // The set of actions present in this program.
+            std::set<uint_fast64_t> actionIndices;
             
             // A map of actions to the set of modules containing commands labelled with this action.
-            std::map<std::string, std::set<uint_fast64_t>> actionsToModuleIndexMap;
+            std::map<uint_fast64_t, std::set<uint_fast64_t>> actionIndicesToModuleIndexMap;
             
             // A mapping from variable names to the modules in which they were declared.
             std::map<std::string, uint_fast64_t> variableToModuleIndexMap;
diff --git a/src/storage/prism/TransitionReward.cpp b/src/storage/prism/TransitionReward.cpp
index 4b6332b91..2fee8dabc 100644
--- a/src/storage/prism/TransitionReward.cpp
+++ b/src/storage/prism/TransitionReward.cpp
@@ -2,12 +2,16 @@
 
 namespace storm {
     namespace prism {
-        TransitionReward::TransitionReward(std::string const& commandName, storm::expressions::Expression const& statePredicateExpression, storm::expressions::Expression const& rewardValueExpression, std::string const& filename, uint_fast64_t lineNumber) : LocatedInformation(filename, lineNumber), commandName(commandName), statePredicateExpression(statePredicateExpression), rewardValueExpression(rewardValueExpression) {
+        TransitionReward::TransitionReward(uint_fast64_t actionIndex, std::string const& actionName, storm::expressions::Expression const& statePredicateExpression, storm::expressions::Expression const& rewardValueExpression, std::string const& filename, uint_fast64_t lineNumber) : LocatedInformation(filename, lineNumber), actionName(actionName), labeled(actionName != ""), statePredicateExpression(statePredicateExpression), rewardValueExpression(rewardValueExpression) {
             // Nothing to do here.
         }
         
         std::string const& TransitionReward::getActionName() const {
-            return this->commandName;
+            return this->actionName;
+        }
+        
+        uint_fast64_t TransitionReward::getActionIndex() const {
+            return this->actionIndex;
         }
         
         storm::expressions::Expression const& TransitionReward::getStatePredicateExpression() const {
@@ -18,8 +22,12 @@ namespace storm {
             return this->rewardValueExpression;
         }
         
+        bool TransitionReward::isLabeled() const {
+            return labeled;
+        }
+        
         TransitionReward TransitionReward::substitute(std::map<storm::expressions::Variable, storm::expressions::Expression> const& substitution) const {
-            return TransitionReward(this->getActionName(), this->getStatePredicateExpression().substitute(substitution), this->getRewardValueExpression().substitute(substitution), this->getFilename(), this->getLineNumber());
+            return TransitionReward(this->getActionIndex(), this->getActionName(), this->getStatePredicateExpression().substitute(substitution), this->getRewardValueExpression().substitute(substitution), this->getFilename(), this->getLineNumber());
         }
         
         std::ostream& operator<<(std::ostream& stream, TransitionReward const& transitionReward) {
diff --git a/src/storage/prism/TransitionReward.h b/src/storage/prism/TransitionReward.h
index 91d12b941..4d693b899 100644
--- a/src/storage/prism/TransitionReward.h
+++ b/src/storage/prism/TransitionReward.h
@@ -16,6 +16,7 @@ namespace storm {
              * Creates a transition reward for the transitions with the given name emanating from states satisfying the
              * given expression with the value given by another expression.
              *
+             * @param actionIndex The index of the action.
              * @param actionName The name of the command that obtains this reward.
              * @param statePredicateExpression The predicate that needs to hold before taking a transition with the previously
              * specified name in order to obtain the reward.
@@ -23,7 +24,7 @@ namespace storm {
              * @param filename The filename in which the transition reward is defined.
              * @param lineNumber The line number in which the transition reward is defined.
              */
-            TransitionReward(std::string const& actionName, storm::expressions::Expression const& statePredicateExpression, storm::expressions::Expression const& rewardValueExpression, std::string const& filename = "", uint_fast64_t lineNumber = 0);
+            TransitionReward(uint_fast64_t actionIndex, std::string const& actionName, storm::expressions::Expression const& statePredicateExpression, storm::expressions::Expression const& rewardValueExpression, std::string const& filename = "", uint_fast64_t lineNumber = 0);
             
             // Create default implementations of constructors/assignment.
             TransitionReward() = default;
@@ -41,6 +42,13 @@ namespace storm {
              */
             std::string const& getActionName() const;
             
+            /*!
+             * Retrieves the action index of the action associated with this transition reward (if any).
+             *
+             * @return The action index of the transition reward.
+             */
+            uint_fast64_t getActionIndex() const;
+            
             /*!
              * Retrieves the state predicate expression that is associated with this state reward.
              *
@@ -54,6 +62,13 @@ namespace storm {
              * @return The reward value expression associated with this state reward.
              */
             storm::expressions::Expression const& getRewardValueExpression() const;
+
+            /*!
+             * Retrieves whether the transition reward has an action label.
+             *
+             * @return True iff the transition reward has an action label.
+             */
+            bool isLabeled() const;
             
             /*!
              * Substitutes all identifiers in the transition reward according to the given map.
@@ -66,8 +81,14 @@ namespace storm {
             friend std::ostream& operator<<(std::ostream& stream, TransitionReward const& transitionReward);
 
         private:
+            // The index of the action name.
+            uint_fast64_t actionIndex;
+            
             // The name of the command this transition-based reward is attached to.
-            std::string commandName;
+            std::string actionName;
+            
+            // A flag that stores whether the transition reward has an action label.
+            bool labeled;
             
             // A predicate that needs to be satisfied by states for the reward to be obtained (by taking
             // a corresponding command transition).
diff --git a/src/utility/PrismUtility.h b/src/utility/PrismUtility.h
index e42a83107..e54932538 100644
--- a/src/utility/PrismUtility.h
+++ b/src/utility/PrismUtility.h
@@ -13,7 +13,7 @@ namespace storm {
             template<typename ValueType, typename KeyType=uint_fast64_t, typename Compare=std::less<uint_fast64_t>>
             struct Choice {
             public:
-                Choice(std::string const& actionLabel) : distribution(), actionLabel(actionLabel), choiceLabels() {
+                Choice(uint_fast64_t actionIndex = 0) : distribution(), actionIndex(actionIndex), choiceLabels() {
                     // Intentionally left empty.
                 }
                 
@@ -108,12 +108,12 @@ namespace storm {
                 }
                 
                 /*!
-                 * Retrieves the action label of this choice.
+                 * Retrieves the index of the action of this choice.
                  *
-                 * @return The action label of this choice.
+                 * @return The index of the action of this choice.
                  */
-                std::string const& getActionLabel() const {
-                    return actionLabel;
+                uint_fast64_t getActionIndex() const {
+                    return actionIndex;
                 }
                 
                 /*!
@@ -152,8 +152,8 @@ namespace storm {
                 // The distribution that is associated with the choice.
                 std::map<KeyType, ValueType, Compare> distribution;
                 
-                // The label of the choice.
-                std::string actionLabel;
+                // The index of the action name.
+                uint_fast64_t actionIndex;
                 
                 // The labels that are associated with this choice.
                 boost::container::flat_set<uint_fast64_t> choiceLabels;