Thanks for the reply Cetin.

The reason I used macro subst is because the workbooks the information is being appended from may have more than one worksheet. I generally open the workbook up to get the sheet names into an array and append the data from each sheet.
I've a suspicion that the problem I've been encountering may be caused by the program trying to proceed before the instance of excel has had a chance to close. Therefore it is still open when it tries to append the data from the workbook.

Lee.