JDK6 includes web services support, which conflicts/interferes with the web services built into JBoss AS. The JDK6 variant of 4.2.3 fixes that problem (see http://www.jboss.com/index.html?module=bb&op=viewtopic&t=102355 for one of many discussion of that topic). In addition, the JDK6 variant of 4.2.3 is compiled with the JDK 6 compiler. Other than that, they are the same. JDK 6 has better performance than JDK 5. View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4203035#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...