Index / Match Fucntion Failure

warrevar 

Well, in general I'd trust Excel to be doing the comparisons correctly.

I'd look for discrepancies in the two sets of data. Have you tried doing the comparisons with the data in each of the 30 columns without doing that concatenation? If there are discrepancies, they're going to originate in a column, so they'd be more readily tracked down by comparing at that level.

And time and again the discrepancies in a case like this can be trailing spaces--invisible to the human eye but glaringly obvious to the computer. Or special characters, or numbers that are text in one case but numbers in another.

Anyway, my main point is to assume that there is a discrepancy, not to assume that Excel is failing.

Is it possible for you to attach a copy of the spreadsheet, or a representative portion of it? Without that, all we can do is offer generalizations, such as I've done above