The PowerPivot engine itself I imagine is in unmanaged code, but the code that just writes datasets and whatnot to the model is (from what I've heard)managed code, and indeed you can decmpile the libraries and see all sorts of things, I've only looked around for about 10 minutes or so. And I had just read on some obscure thread that someone had successfully found the undocumented API call to write to the model, which is what I'm wanting to do - but I don't even know what the product name is that supposedly does this, sorry.