The reason might be easier to diagnose, if you were to send the line of code that triggers this error. Generally speaking, however, IDL cannot change a structure definition during Runtime. If your current IDL session (or your currently-running IDL routine) has a current definition of {VERSIONMAJOR}, then you either have to do a .FULL_RESET_SESSION to clear it in your current IDL session, or you have to exit and restart IDL. I don't recognize this exact error message. But, if I received such a message, the first thing I would do is to enter "HELP, {VERSIONMAJOR}, /STRUCT" at the IDL> command line. This would show the field names and their default values of the {VERSIONMAJOR} definition that I was not allowed to change. That would hopefully, then, help me deduce the source from which this conflicting definition originated. James Jones