Thanks for the reference - though I had to dig out an old copy of IE to read it, it doesn't seem to display in Firefox. This seems to be very close to the JAXP design, with the classes XmlInput and XmlOutput corresponding to JAXPs Source and Result. I'm not sure it solves the underlying problem of extensibility: again it seems to be just a union type. The bigger problem with the MVP interface is that there is no "Transformer" object: the IXmlTransform is a compiled stylesheet, not a loaded stylesheet, so there is nowhere to initialize aspects of the dynamic context or initialization parameters such as initial mode and initial named template, other than in the method call to Transform() itself, which then becomes unwieldy as you add more parameters. One way round that, of course, would be to replace the "parameters" parameter to Transform() with a "dynamicContext" parameter.