Fluorescence confocal optical microscopy (FCOM) was used to characterize MFI membranes made by secondary (seeded) growth. It is demonstrated that FCOM is a powerful tool for the non-destructive evaluation of zeolite membranes. This technique can provide two-dimensional, optical sections of the membranes with submicron resolution without physical damage to the sample. When contrasted with fluorescein-Na salt, it is shown that the grain boundary network in MFI membranes can be observed along the thickness of the membranes. Moreover, defects that do not propagate to the membrane surface acid, therefore, are not detected by SEM, can be clearly imaged. One example of such a defect are internal cracks in the membrane.