Sorry about that. From all I've been able to find, Excel 2011 should be able to open files in the earlier formats without difficulty. So, unless there's a problem with your Excel installation, the implication is that there's a problem with the files. Since you're able to open the same files on a PC, that seems to rule out the latter. Perhaps you should try repairing your Office installation. As I don't use a Mac, I don't know the details of that process.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]