I have some more information about this problem. The problem only occurs when an xa-datasource backs an entity bean the classes of which are deployed in a .jar but the deployment descriptor of which is deployed in an .ear. Note: the reason for this strange situation is that we are using JBoss Messaging with JBoss 4.x and in order to be able to send or receive messages we have to specify the class loader, and the only way to do that is via a jboss-app.xml in an application (.ear) bundle. However (some of) the supporting classes must be available to other applications/jars so we package them separately. View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4028429#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...