Can you post that entire exception stacktrace? I want to see what code in JBoss AS (is it the AS6 timer service?) is using Hibernate. If it's the timerservice, then you specify the dialect in JBOSS_HOME/common/deploy/jboss-ejb3-timerservice-mk2.jar/META-INF/persistence.xml (since you mention you are using JBoss AS6 SNAPSHOT, that's the correct location).