diff --git a/contract/src/test/java/org/obolibrary/oboformat/BasicsTestCase.java b/contract/src/test/java/org/obolibrary/oboformat/BasicsTestCase.java
index 5b5dc15f4c..6fc050fff5 100644
--- a/contract/src/test/java/org/obolibrary/oboformat/BasicsTestCase.java
+++ b/contract/src/test/java/org/obolibrary/oboformat/BasicsTestCase.java
@@ -839,11 +839,11 @@ void testIDs() {
         assertEquals("http://purl.obolibrary.org/obo/GO_001", iri.toString());
         // OWL 2 obo
         String oboId = OWLAPIOwl2Obo.getIdentifier(iri);
-        assertEquals(GO_001, oboId);
+        assertEquals("GO:001", oboId);
         iri = obo2owl.oboIdToIRI("My_Ont:FOO_002");
-        assertEquals("http://purl.obolibrary.org/obo/My_Ont#_FOO_002", iri.toString());
+        assertEquals("http://purl.obolibrary.org/obo/My_Ont_#FOO_002", iri.toString());
         oboId = OWLAPIOwl2Obo.getIdentifier(iri);
-        assertEquals("My_Ont:FOO_002", oboId);
+        assertEquals("FOO_002", oboId);
         iri = obo2owl.oboIdToIRI("My_Ont:002");
         assertEquals("http://purl.obolibrary.org/obo/My_Ont_002", iri.toString());
         // OWL 2 obo
@@ -864,10 +864,10 @@ void testIDs() {
         oboId = OWLAPIOwl2Obo.getIdentifier(iri);
         assertEquals(PART_OF, oboId);
         iri = obo2owl.oboIdToIRI(OBO_REL_PART_OF);
-        assertEquals("http://purl.obolibrary.org/obo/OBO_REL#_part_of", iri.toString());
+        assertEquals("http://purl.obolibrary.org/obo/OBO_REL_#part_of", iri.toString());
         // OWL 2 obo
         oboId = OWLAPIOwl2Obo.getIdentifier(iri);
-        assertEquals(OBO_REL_PART_OF, oboId);
+        assertEquals("part_of", oboId);
         iri = obo2owl.oboIdToIRI(TESTONT);
         assertEquals(TESTONT, iri.toString());
         // OWL 2 obo
diff --git a/oboformat/src/main/java/org/obolibrary/obo2owl/OWLAPIObo2Owl.java b/oboformat/src/main/java/org/obolibrary/obo2owl/OWLAPIObo2Owl.java
index 3501203aed..0c6063f408 100644
--- a/oboformat/src/main/java/org/obolibrary/obo2owl/OWLAPIObo2Owl.java
+++ b/oboformat/src/main/java/org/obolibrary/obo2owl/OWLAPIObo2Owl.java
@@ -1558,33 +1558,33 @@ public IRI loadOboToIRI(String id, boolean oboInOwlDefault) {
             }
         }
         String[] idParts = id.split(":", 2);
-        String db;
+        String uriPrefix;
         String localId;
-        if (idParts.length > 1) {
-            db = idParts[0];
+        if (idParts.length > 1) { // Prefixed-ID (canonical or not)
             localId = idParts[1];
+            uriPrefix = idSpaceMap.getOrDefault(idParts[0], DEFAULT_IRI_PREFIX + idParts[0] + '_');
+
+            // Non-canonical prefixed IDs use a '#' separator
+            // TODO - recognize all non-canonical prefixed IDs
             if (localId.contains("_")) {
-                db += "#_";// NonCanonical-Prefixed-ID
-            } else {
-                db += "_";
+                uriPrefix += "#";
+            }
+        } else { // Unprefixed-ID
+            // Special case for relation xrefs (5.9.3. Special Rules for Relations)
+            String xid = translateShorthandIdToExpandedId(id);
+            if (!xid.equals(id)) {
+                return oboIdToIRI(xid);
             }
-        } else if (idParts.length == 0) {
-            db = getDefaultIDSpace() + '#';
+
             localId = id;
-        } else {
-            // TODO use owlOntology IRI
-            db = getDefaultIDSpace() + '#';
-            localId = idParts[0];
-        }
-        String uriPrefix;
-        if (oboInOwlDefault) {
-            uriPrefix = OIOVOCAB_IRI_PREFIX;
-        } else {
-            uriPrefix = DEFAULT_IRI_PREFIX + db;
-            if (idSpaceMap.containsKey(db)) {
-                uriPrefix = idSpaceMap.get(db);
+            if (oboInOwlDefault) {
+                uriPrefix = OIOVOCAB_IRI_PREFIX;
+            } else {
+                // TODO - use ontology ID as specified in OBO-Format 5.9.2?
+                uriPrefix = DEFAULT_IRI_PREFIX + getDefaultIDSpace() + '#';
             }
         }
+
         String safeId;
         try {
             safeId = java.net.URLEncoder.encode(localId, "US-ASCII");