OK I have found a solution! The deadlocking seems to be caused by this bug: https://jira.jboss.org/jira/browse/JBAS-5801 As JBoss AS 4.2.4.GA is not available yet, I manually backported the fixes from the 4.2.4.GA branch into my JBoss 4.2.3.GA sources (building and updating the jms-ra.rar and jboss-xa-jdbc.rar archives accordingly): http://fisheye.jboss.org/changelog/JBossAS/?cs=76314 This seems to have fixed the IllegalMonitorStateException problem and has stopped my MDBs from deadlocking. We are now looking forward to an official JBoss 4.2.4.GA release so we don't have to ship with a patched server... View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4186571#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...