I am trying to use the Sun reference implementation of JAXB version 1.0.2 in JBoss 3.2.1. When trying to create a JAXBContext I get a JAXBException caused by a NoClassDefFoundError on javax.xml.namespace.NamespaceContext. This class is in namespace.jar from the Web Services Development Pack 1.3. I have tried putting this jar on the initial classpath at JBoss startup and in the server/default/lib directory in JBoss, but get the exception in both cases. The only way I have got it to work is to put the namespace.jar file in the endorsed directory of the JRE. I have seen a similar post related to javax.xml.namespace.QName that led me to believe it may be class loader related. These two classes are in the same package but in different jar files. Has anyone seen this before or have a better solution than either repackaging the Sun implementation or putting the namespace.jar file in the endorsed directory?