Hi Sam:

IYO would it be faster to use the current system with two workbooks?
I used an addin in the application that I wrote, but I have no comparison with using two instantiations of EXCEL. At a guess I wouldn't think it would make much difference. I would hope that EXCEL would work out the multi threading as best it could either way.

I think the way you have designed it; keeping everything in memory, gives this a good chance of working either way .

I am still not sure whether a "free for all" with all the objects competing for processor time is the optimal way of organising this, or whether some sort of scheduler is going to be necessary. Is there going to be some way fo preventing one UDO getting priority over the the others. We don't need to recalculate everything all the time. It this going to be part of the EXCEL Helper??