Use controls presence. If use="required" the attribute must appear in the XML. Fixed is a value constraint. If the attribute is present and different from the fixed value the parsing will fail. All this stuff is handled by the underlying xml parser, which is xerces SAX in our case. Xerces will include fixed and default attributes into org.xml.sax.Attributes only if schema validation is enabled. In XB you can do that by unmarshaller.setSchemaValidation(true); View the original post : http://www.jboss.org/index.html?module=bb&op=viewtopic&p=4261866#... Reply to the post : http://www.jboss.org/index.html?module=bb&op=posting&mode=reply&a...