diff --git a/prism/src/explicit/POMDPSimple.java b/prism/src/explicit/POMDPSimple.java
index 32a06d76..9563f271 100644
--- a/prism/src/explicit/POMDPSimple.java
+++ b/prism/src/explicit/POMDPSimple.java
@@ -40,6 +40,12 @@ import prism.PrismUtils;
 /**
  * Simple explicit-state representation of a POMDP.
  * Basically a {@link MDPSimple} with observability info.
+ * <br><br>
+ * POMDPs require that states with the same observation have
+ * the same set of available actions. This class further requires
+ * that these actions appear in the same order (in terms of
+ * choice indexing) in each equivalent state. This is enforced
+ * when calling setObservation().
  */
 public class POMDPSimple extends MDPSimple implements POMDP
 {
@@ -261,29 +267,63 @@ public class POMDPSimple extends MDPSimple implements POMDP
 		// Otherwise, check that the actions for existing states with
 		// the same observation match this one
 		else {
-			// Get and sort action strings for existing state(s)
-			List<String> observationStateActions = new ArrayList<>();
-			int numChoices = getNumChoices(observationState);
-			for (int i = 0; i < numChoices; i++) {
-				Object action = getAction(observationState, i);
-				observationStateActions.add(action == null ? "" : action.toString());
-			}
-	        Collections.sort(observationStateActions);
-			// Get and sort action strings for the new state
-			List<String> sActions = new ArrayList<>();
-			numChoices = getNumChoices(s);
-			for (int i = 0; i < numChoices; i++) {
-				Object action = getAction(s, i);
-				sActions.add(action == null ? "" : action.toString());
-			}
-	        Collections.sort(sActions);
-	        // Check match
-			if (!(observationStateActions.equals(sActions))) {
-				throw new PrismException("Differing actions found in states: " + observationStateActions + " vs. " + sActions);
+			checkActionsMatchExactly(s, observationState);
+		}
+	}
+	
+	/**
+	 * Check that the available actions and their ordering
+	 * in states s1 and s2 match, and throw an exception if not.
+	 */
+	protected void checkActionsMatchExactly(int s1, int s2) throws PrismException
+	{
+		int numChoices = getNumChoices(s1);
+		if (numChoices != getNumChoices(s2)) {
+			throw new PrismException("Differing actions found in states: " + getAvailableActions(s1) + " vs. " + getAvailableActions(s2));
+		}
+		for (int i = 0; i < numChoices; i++) {
+			Object action1 = getAction(s1, i);
+			Object action2 = getAction(s2, i);
+			if (action1 == null) {
+				if (action2 != null) {
+					throw new PrismException("Differing actions found in states: " + getAvailableActions(s1) + " vs. " + getAvailableActions(s2));
+				}
+			} else {
+				if (!action1.equals(action2)) {
+					throw new PrismException("Differing actions found in states: " + getAvailableActions(s1) + " vs. " + getAvailableActions(s2));
+				}
 			}
 		}
 	}
 	
+	/**
+	 * Check that the *sets* of available actions in states s1 and s2 match,
+	 * and throw an exception if not.
+	 */
+	protected void checkActionsMatch(int s1, int s2) throws PrismException
+	{
+		// Get and sort action strings for s1
+		List<String> s1Actions = new ArrayList<>();
+		int numChoices = getNumChoices(s1);
+		for (int i = 0; i < numChoices; i++) {
+			Object action = getAction(s1, i);
+			s1Actions.add(action == null ? "" : action.toString());
+		}
+        Collections.sort(s1Actions);
+		// Get and sort action strings for s2
+		List<String> s2Actions = new ArrayList<>();
+		numChoices = getNumChoices(s2);
+		for (int i = 0; i < numChoices; i++) {
+			Object action = getAction(s2, i);
+			s2Actions.add(action == null ? "" : action.toString());
+		}
+        Collections.sort(s2Actions);
+        // Check match
+		if (!(s1Actions.equals(s2Actions))) {
+			throw new PrismException("Differing actions found in states: " + s1Actions + " vs. " + s2Actions);
+		}
+	}
+	
 	// Accessors (for PartiallyObservableModel)
 	
 	@Override