By "XML based string" I assume you mean that the string contains a textual representation of an XML document. If that's correct, where do you see the difference between a WS that returns a string (which you have) and a WS that returns such an "XML based string"? Both sound identical to me. You would have to encapsulate the XML string so that it doesn't interfere with the
SOAP processing (assuming you're talking about a SOAP WS - you haven't said), possibly by base-64 encoding it.