On JBoss AS 7.1.1, with the following service implementation:

@WebService(...)
@javax.xml.ws.BindingType (value=javax.xml.ws.soap.SOAPBinding.SOAP12HTTP_MTOM_BINDING)
public class SEIImpl {
...
public DataHandler getBin() {...} /* maps to a type="xs:base64Binary" in wsdl */
public List<String> getString(){...} /* maps to sequence of String in wsdl */
...
}

With the default jbossws-cxf, the responses are:

• for getString() method
  Content-Type: multipart/related; type="application/xop+xml"; ...
• for getBin() method
  Content-Type: multipart/related; type="application/xop+xml"; ...

-> both responses are MTOM/XOP modified ! The getString() response should not.

With jbossws-native (after the jbossws stack switching) , the responses are the expected ones:

• for getString() method
  Content-Type: text/xml;charset=UTF-8 <env:Envelope .... text content ..../env:Envelope>
• for getBin() method
  Content-Type: multipart/related; type="application/xop+xml"; ...

Best regards,

Note: same behaviour with @javax.xml.ws.soap.MTOM or SOAP11HTTP_MTOM_BINDING