For what its worth (your post is 2 years old now, but I found it on a search so maybe someone else will find it as well...) I am having the same problem with jboss 4.0.3. My code is almost identical -- lookup, get datasource, then ClassCastException on ds.getConnection(). My exception says "ClassCastException: com.inet.tds.TdsDriver" (the name of my JDBC driver). However, I have created two datasources that use the same driver, but one is an XA connection and the other is not. The non-XA connection was created by modifying the jboss-4.0.3/examples/jca/mssql-ds.xml and this one works fine. The XA connection was created by modifying examples/jca/mssql-xa-ds.xml and this one gives the problem. So I think the problem has to do with the classloading context of the connection pools in JBoss. From the XML code above, looks like the original poster also was using an XA datasource and got the same error. View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=3966147#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...