--- a/src/test/java/org/sbaresearch/owl/OwlApiFacadeTest.java
+++ b/src/test/java/org/sbaresearch/owl/OwlApiFacadeTest.java
@@ -31,6 +31,7 @@
 import org.semanticweb.owlapi.vocab.OWL2Datatype;
 import uk.ac.manchester.cs.owl.owlapi.OWL2DatatypeImpl;
 
+import java.io.ByteArrayInputStream;
 import java.io.File;
 import java.io.IOException;
 import java.lang.reflect.InvocationTargetException;
@@ -816,6 +817,21 @@
         assertFalse(owlApi.isIndividualOfClass(indiv, IRI.create("http://localhost/test_ab4.owl#ClassE")));
     }
 
+    @Test(expected = ConstraintsViolatedException.class)
+    public void testToXMLString_ontologyNull_shouldThrowException() throws IOException, OWLOntologyStorageException {
+        OwlApiFacade.toXMLString(null);
+    }
+
+    @Test
+    public void testToXMLString_validOntology_shouldReturnOntologyAsXML() throws IOException, OWLOntologyStorageException, OWLOntologyCreationException {
+        OwlApiFacade expected = TestUtils.createDefaultOwlApiFacade();
+        expected.load(new ByteArrayInputStream(TestResources.testOntology.getBytes("UTF-8")));
+        String xmlString = OwlApiFacade.toXMLString(expected.getOntology());
+        OwlApiFacade actual = TestUtils.createDefaultOwlApiFacade();
+        actual.load(new ByteArrayInputStream(xmlString.getBytes("UTF-8")));
+        assertThat(actual.getOntology().getAxiomCount(), equalTo(expected.getOntology().getAxiomCount()));
+    }
+
     //<editor-fold desc="helper methods">
 
     private String getOwlUri() throws URISyntaxException {