diff --git a/prism/src/explicit/ProbModelChecker.java b/prism/src/explicit/ProbModelChecker.java
index 9e8c23ea..a891bf34 100644
--- a/prism/src/explicit/ProbModelChecker.java
+++ b/prism/src/explicit/ProbModelChecker.java
@@ -875,12 +875,26 @@ public class ProbModelChecker extends NonProbModelChecker
 	}
 
 	/**
-	 * Construct rewards from a reward structure and a model.
+	 * Construct rewards from a (non-negative) reward structure and a model.
 	 */
 	protected Rewards constructRewards(Model model, RewardStruct rewStruct) throws PrismException
+	{
+		return constructRewards(model, rewStruct, false);
+	}
+
+	/**
+	 * Construct rewards from a reward structure and a model.
+	 * <br>
+	 * If {@code allowNegativeRewards} is true, the rewards may be positive and negative, i.e., weights.
+	 */
+	protected Rewards constructRewards(Model model, RewardStruct rewStruct, boolean allowNegativeRewards) throws PrismException
 	{
 		Rewards rewards;
 		ConstructRewards constructRewards = new ConstructRewards(mainLog);
+
+		if (allowNegativeRewards)
+			constructRewards.allowNegativeRewards();
+
 		switch (model.getModelType()) {
 		case CTMC:
 		case DTMC:
@@ -894,7 +908,7 @@ public class ProbModelChecker extends NonProbModelChecker
 		}
 		return rewards;
 	}
-	
+
 	/**
 	 * Compute rewards for the contents of an R operator.
 	 */
diff --git a/prism/src/explicit/rewards/ConstructRewards.java b/prism/src/explicit/rewards/ConstructRewards.java
index b163ad31..401936c7 100644
--- a/prism/src/explicit/rewards/ConstructRewards.java
+++ b/prism/src/explicit/rewards/ConstructRewards.java
@@ -50,6 +50,9 @@ public class ConstructRewards
 {
 	protected PrismLog mainLog;
 
+	/** Allow negative rewards, i.e., weights. Defaults to false. */
+	protected boolean allowNegative = false;
+
 	public ConstructRewards()
 	{
 		this(new PrismFileLog("stdout"));
@@ -60,6 +63,12 @@ public class ConstructRewards
 		this.mainLog = mainLog;
 	}
 
+	/** Set flag that negative rewards are allowed, i.e., weights */
+	public void allowNegativeRewards()
+	{
+		allowNegative = true;
+	}
+
 	/**
 	 * Construct rewards from a model and reward structure. 
 	 * @param model The model
@@ -100,6 +109,8 @@ public class ConstructRewards
 			double rew = rewStr.getReward(0).evaluateDouble(constantValues);
 			if (Double.isNaN(rew))
 				throw new PrismLangException("Reward structure evaluates to NaN (at any state)", rewStr.getReward(0));
+			if (!allowNegative && rew < 0)
+				throw new PrismLangException("Reward structure evaluates to " + rew + " (at any state), negative rewards not allowed", rewStr.getReward(0));
 			return new StateRewardsConstant(rew);
 		}
 		// Normal: state rewards
@@ -115,6 +126,8 @@ public class ConstructRewards
 						double rew = rewStr.getReward(i).evaluateDouble(constantValues, statesList.get(j));
 						if (Double.isNaN(rew))
 							throw new PrismLangException("Reward structure evaluates to NaN at state " + statesList.get(j), rewStr.getReward(i));
+						if (!allowNegative && rew < 0)
+							throw new PrismLangException("Reward structure evaluates to " + rew + " at state " + statesList.get(j) +", negative rewards not allowed", rewStr.getReward(i));
 						rewSA.addToStateReward(j, rew);
 					}
 				}
@@ -142,6 +155,8 @@ public class ConstructRewards
 			double rew = rewStr.getReward(0).evaluateDouble(constantValues);
 			if (Double.isNaN(rew))
 				throw new PrismLangException("Reward structure evaluates to NaN (at any state)", rewStr.getReward(0));
+			if (!allowNegative && rew < 0)
+				throw new PrismLangException("Reward structure evaluates to " + rew + " (at any state), negative rewards not allowed", rewStr.getReward(0));
 			return new StateRewardsConstant(rew);
 		}
 		// Normal: state and transition rewards
@@ -165,6 +180,8 @@ public class ConstructRewards
 									double rew = rewStr.getReward(i).evaluateDouble(constantValues, statesList.get(j));
 									if (Double.isNaN(rew))
 										throw new PrismLangException("Reward structure evaluates to NaN at state " + statesList.get(j), rewStr.getReward(i));
+									if (!allowNegative && rew < 0)
+										throw new PrismLangException("Reward structure evaluates to " + rew + " at state " + statesList.get(j) +", negative rewards not allowed", rewStr.getReward(i));
 									rewSimple.addToTransitionReward(j, k, rew);
 								}
 							}
@@ -174,6 +191,8 @@ public class ConstructRewards
 							double rew = rewStr.getReward(i).evaluateDouble(constantValues, statesList.get(j));
 							if (Double.isNaN(rew))
 								throw new PrismLangException("Reward structure evaluates to NaN at state " + statesList.get(j), rewStr.getReward(i));
+							if (!allowNegative && rew < 0)
+								throw new PrismLangException("Reward structure evaluates to " + rew + " at state " + statesList.get(j) +", negative rewards not allowed", rewStr.getReward(i));
 							rewSimple.addToStateReward(j, rew);
 						}
 					}
@@ -191,21 +210,18 @@ public class ConstructRewards
 	 */
 	public MCRewards buildMCRewardsFromPrismExplicit(DTMC mc, File rews, File rewt) throws PrismException
 	{
-		BufferedReader in;
 		String s, ss[];
 		int i, lineNum = 0;
 		double reward;
 		StateRewardsArray rewSA = new StateRewardsArray(mc.getNumStates());
 
-		try {
-			if (rews != null) {
-				// Open state rewards file
-				in = new BufferedReader(new FileReader(rews));
+		if (rews != null) {
+			// Open state rewards file, automatic close
+			try (BufferedReader in = new BufferedReader(new FileReader(rews))) {
 				// Ignore first line
 				s = in.readLine();
 				lineNum = 1;
 				if (s == null) {
-					in.close();
 					throw new PrismException("Missing first line of state rewards file");
 				}
 				// Go though list of state rewards in file
@@ -217,18 +233,19 @@ public class ConstructRewards
 						ss = s.split(" ");
 						i = Integer.parseInt(ss[0]);
 						reward = Double.parseDouble(ss[1]);
+						if (!allowNegative && reward < 0) {
+							throw new PrismLangException("Found state reward " + reward + " at state " + i +", negative rewards not allowed");
+						}
 						rewSA.setStateReward(i, reward);
 					}
 					s = in.readLine();
 					lineNum++;
 				}
-				// Close file
-				in.close();
+			} catch (IOException e) {
+				throw new PrismException("Could not read state rewards from file \"" + rews + "\"" + e);
+			} catch (NumberFormatException e) {
+				throw new PrismException("Problem in state rewards file (line " + lineNum + ") for MDP");
 			}
-		} catch (IOException e) {
-			throw new PrismException("Could not read state rewards from file \"" + rews + "\"" + e);
-		} catch (NumberFormatException e) {
-			throw new PrismException("Problem in state rewards file (line " + lineNum + ") for MDP");
 		}
 
 		if (rewt != null) {
@@ -246,21 +263,18 @@ public class ConstructRewards
 	 */
 	public MDPRewards buildMDPRewardsFromPrismExplicit(MDP mdp, File rews, File rewt) throws PrismException
 	{
-		BufferedReader in;
 		String s, ss[];
 		int i, j, lineNum = 0;
 		double reward;
 		MDPRewardsSimple rs = new MDPRewardsSimple(mdp.getNumStates());
 
-		try {
-			if (rews != null) {
-				// Open state rewards file
-				in = new BufferedReader(new FileReader(rews));
+		if (rews != null) {
+			// Open state rewards file, automatic close
+			try (BufferedReader in = new BufferedReader(new FileReader(rews))) {
 				// Ignore first line
 				s = in.readLine();
 				lineNum = 1;
 				if (s == null) {
-					in.close();
 					throw new PrismException("Missing first line of state rewards file");
 				}
 				// Go though list of state rewards in file
@@ -272,29 +286,28 @@ public class ConstructRewards
 						ss = s.split(" ");
 						i = Integer.parseInt(ss[0]);
 						reward = Double.parseDouble(ss[1]);
+						if (!allowNegative && reward < 0) {
+							throw new PrismLangException("Found state reward " + reward + " at state " + i +", negative rewards not allowed");
+						}
 						rs.setStateReward(i, reward);
 					}
 					s = in.readLine();
 					lineNum++;
 				}
-				// Close file
-				in.close();
+			} catch (IOException e) {
+				throw new PrismException("Could not read state rewards from file \"" + rews + "\"" + e);
+			} catch (NumberFormatException e) {
+				throw new PrismException("Problem in state rewards file (line " + lineNum + ") for MDP");
 			}
-		} catch (IOException e) {
-			throw new PrismException("Could not read state rewards from file \"" + rews + "\"" + e);
-		} catch (NumberFormatException e) {
-			throw new PrismException("Problem in state rewards file (line " + lineNum + ") for MDP");
 		}
 
-		try {
-			if (rewt != null) {
-				// Open transition rewards file
-				in = new BufferedReader(new FileReader(rewt));
+		if (rewt != null) {
+			// Open transition rewards file, automatic close
+			try (BufferedReader in = new BufferedReader(new FileReader(rewt))) {
 				// Ignore first line
 				s = in.readLine();
 				lineNum = 1;
 				if (s == null) {
-					in.close();
 					throw new PrismException("Missing first line of transition rewards file");
 				}
 				// Go though list of transition rewards in file
@@ -307,20 +320,23 @@ public class ConstructRewards
 						i = Integer.parseInt(ss[0]);
 						j = Integer.parseInt(ss[1]);
 						reward = Double.parseDouble(ss[3]);
+						if (!allowNegative && reward < 0) {
+							throw new PrismLangException("Found transition reward " + reward + " at state " + i +", action " + j +", negative rewards not allowed");
+						}
 						rs.setTransitionReward(i, j, reward);
 					}
 					s = in.readLine();
 					lineNum++;
 				}
-				// Close file
-				in.close();
+
+			} catch (IOException e) {
+				throw new PrismException("Could not read transition rewards from file \"" + rewt + "\"" + e);
+			} catch (NumberFormatException e) {
+				throw new PrismException("Problem in transition rewards file (line " + lineNum + ") for MDP");
 			}
-		} catch (IOException e) {
-			throw new PrismException("Could not read transition rewards from file \"" + rewt + "\"" + e);
-		} catch (NumberFormatException e) {
-			throw new PrismException("Problem in transition rewards file (line " + lineNum + ") for MDP");
 		}
 
 		return rs;
 	}
+
 }