Tim, Thanks. I have posted a message on the JCA forum. Please note I may not be the only one seeing this. This JIRA seems to indicate they see it intermittently too: https://jira.jboss.org/jira/browse/JBESB-2484 It says "JBoss Messaging sometimes causes a HeuristicMixedException to be raised by the transaction manager during commit. Once this happens the associated session would appear to be destabilised in that subsequent use does not appear possible... The exceptions are always generated with the same session id which would suggest that there is an underlying issue in the JBoss Messaging codebase. We need to handle this until the issue is resolved within JBM." Their JIRA is more concerned with working around the problem, but it was only opened in March 2009. Perhaps you could ask the ESB guys about it? View the original post : http://www.jboss.org/index.html?module=bb&op=viewtopic&p=4241310#... Reply to the post : http://www.jboss.org/index.html?module=bb&op=posting&mode=reply&a...