I had the very same error when trying to do a lookup from my code running on Tomcat. I was running Tomcat on my local PC along with JBoss 4.0.3 and it worked fine. When I tried looking up a JBoss instance on another machine though I got this error. Turns out I had older jboss libraries in Tomcat's common/libs that had to be upgraded to the latest versions to serialize properly. View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=3957496#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...