It's ajaxSingle="true" that helps you run your code without following recommendations. In my view a:outputPanel optimizes the AJAX requiest since: anonymous wrote : The component is used for components grouping in the AJAX output area, which offers several additional output opportunities such as inserting of non-present in tree components, saving of transient elements after AJAX request and some others. Reference: http://labs.jboss.com/file-access/default/members/jbossajax4jsf/freezone/... What you do with ajaxSingle is that you submit only the changes to the combo box. I'm actually amazed that A4J manages to update the page correctly. I guess though that it updates the whole page while with a:outputPanel you can update only the second combo box. View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4086748#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...