diff --git a/prism/src/explicit/POMDPSimple.java b/prism/src/explicit/POMDPSimple.java
index bcf77f16..afd83c38 100644
--- a/prism/src/explicit/POMDPSimple.java
+++ b/prism/src/explicit/POMDPSimple.java
@@ -27,6 +27,7 @@
 package explicit;
 
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -51,8 +52,8 @@ public class POMDPSimple extends MDPSimple implements POMDP
 	 * i.e. the Unobservation object corresponding to each unobservation index. */
 	protected List<Unobservation> unobservationsList;
 
-	/** Number of choices for each observation (same in states with that observation) */
-	protected List<Integer> observationNumChoices;
+	/** One state corresponding to each observation (used to look up info about it) */
+	protected List<Integer> observationStates;
 	
 	/** Observable assigned to each state */
 	protected List<Integer> observablesMap;
@@ -88,7 +89,7 @@ public class POMDPSimple extends MDPSimple implements POMDP
 		super(pomdp);
 		observationsList = new ArrayList<>(pomdp.observationsList);
 		unobservationsList = new ArrayList<>(pomdp.unobservationsList);
-		observationNumChoices = new ArrayList<>(pomdp.observationNumChoices);
+		observationStates = new ArrayList<>(pomdp.observationStates);
 		observablesMap = new ArrayList<>(pomdp.observablesMap);
 		unobservablesMap = new ArrayList<>(pomdp.unobservablesMap);
 	}
@@ -102,7 +103,12 @@ public class POMDPSimple extends MDPSimple implements POMDP
 		super(pomdp, permut);
 		observationsList = new ArrayList<>(pomdp.observationsList);
 		unobservationsList = new ArrayList<>(pomdp.unobservationsList);
-		observationNumChoices = new ArrayList<>(pomdp.observationNumChoices);
+		int numObservations = pomdp.getNumObservations();
+		observationStates = new ArrayList<>(numObservations);
+		for (int o = 0; o < numObservations; o++) {
+			int s = pomdp.observationStates.get(o);
+			observationStates.add(s == -1 ? -1 : permut[s]);
+		}
 		observablesMap = new ArrayList<Integer>(getNumStates());
 		unobservablesMap = new ArrayList<Integer>(getNumStates());
 		for (int s = 0; s < numStates; s++) {
@@ -141,7 +147,7 @@ public class POMDPSimple extends MDPSimple implements POMDP
 	{
 		observationsList = new ArrayList<>();
 		unobservationsList = new ArrayList<>();
-		observationNumChoices = new ArrayList<>();
+		observationStates = new ArrayList<>();
 		observablesMap = new ArrayList<>();
 		unobservablesMap = new ArrayList<>();
 	}
@@ -153,7 +159,7 @@ public class POMDPSimple extends MDPSimple implements POMDP
 	{
 		observationsList = new ArrayList<>();
 		unobservationsList = new ArrayList<>();
-		observationNumChoices = new ArrayList<>();
+		observationStates = new ArrayList<>();
 		observablesMap = new ArrayList<>(numStates);
 		unobservablesMap = new ArrayList<>(numStates);
 		for (int i = 0; i < numStates; i++) {
@@ -201,17 +207,24 @@ public class POMDPSimple extends MDPSimple implements POMDP
 	}
 
 	/**
-	 * Set the observation info for a state
+	 * Set the observation info for a state.
+	 * If the actions for existing states with this observation do not match,
+	 * an explanatory exception is thrown (so this should be done after transitions
+	 * have been added to the state).
 	 */
 	public void setObservation(int s, Observation observ, Unobservation unobserv) throws PrismException
 	{
 		int oIndex = observationsList.indexOf(observ);
 		if (oIndex == -1) {
 			observationsList.add(observ);
-			observationNumChoices.add(-1);
+			observationStates.add(-1);
 			oIndex = observationsList.size() - 1;
 		}
-		setObservation(s, oIndex);
+		try {
+			setObservation(s, oIndex);
+		} catch (PrismException e) {
+			throw new PrismException("Problem with observation " + observ + ": " + e.getMessage());
+		}
 		int unobservIndex = unobservationsList.indexOf(unobserv);
 		if (unobservIndex == -1) {
 			unobservationsList.add(unobserv);
@@ -221,24 +234,43 @@ public class POMDPSimple extends MDPSimple implements POMDP
 	}
 	
 	/**
-	 * Assign observation o to state s
-	 * (so observation has already been added to the list)
-	 * Also update (and check) info about number of choices for an observation
-	 * (so should be done after transitions have been added)
+	 * Assign observation with index o to state s.
+	 * (assumes observation has already been added to the list)
+	 * If the actions for existing states with this observation do not match,
+	 * an explanatory exception is thrown (so this should be done after transitions
+	 * have been added to the state).
 	 */
 	protected void setObservation(int s, int o) throws PrismException
 	{
 		// Set observation
 		observablesMap.set(s, o);
-		// Update observation choice counts
-		// and check that this value matches
-		int numChoicesObs = observationNumChoices.get(o);
-		int numChoicesState = getNumChoices(s);
-		if (numChoicesObs == -1) {
-			observationNumChoices.set(o, numChoicesState);
-		} else {
-			if (numChoicesState != numChoicesObs) {
-				throw new PrismException("Conflicting numbers of choices for observation " + getObservation(o) + ": " + numChoicesObs + " vs. " + numChoicesState);
+		// If this is first state with this observation, store its index
+		int observationState = observationStates.get(o);
+		if (observationState == -1) {
+			observationStates.set(o, s);
+		}
+		// 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);
 			}
 		}
 	}
@@ -272,7 +304,7 @@ public class POMDPSimple extends MDPSimple implements POMDP
 	@Override
 	public int getNumChoicesForObservation(int o)
 	{
-		return observationNumChoices.get(o);
+		return getNumChoices(observationStates.get(o));
 	}
 	
 	// Accessors (for POMDP)