More specifically you cannot replace the JBoss Cache 1.x jars in JBoss 4.x if you are using any of the following AS services which depend on the API-incompatible JBC 1.x: 1) HttpSession replication. 2) EJB3 SFSB replication 3) JBC as a Hibernate 2nd Level Cache (which includes EJB3 entities). If you remove those services, you should be able to replace the jars in server/all/lib. But it's probably easier to package JBC in a war. View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4050969#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...