I have searched but yet to find specifics as to what is incompatible with the xlsb format. I work with a fair number of large files and like to save them as binary files .xlsb. Most files save then reopen just fine. Occasionally I get a file where the xlsb version will not reopen. If I try to reopen it I get the message "Microsoft Office Excel has encountered a problem and needs to close." If I could find what is incompatible I could possibly remove that from the file prior to saving it in the binary format. Anyone know what is not compatible with the binary format?