The error is only appearing on the client's system, and it's been very frustrating to be unable to duplicate the problem on my system. Therefore, I am assuming this is causing the problem, because it is the only obvious difference between how the two types of procedures in the application are talking to Excel. The single connection procedure has been running flawlessly.

The client will be trying out the update tomorrow, and I'm hoping he'll have good news for me, because I'm about out of things to try next!

>
>I never had a problem doing this. Two completely separate instances of Excel accessing different spreadsheets in different threads. Unless there is a resource problem (e.g. memory) on the machine, I can't think of a reason for this to occur.