Membranes are becoming increasingly important for performing gas or chemical separations at low energy cost. Defects, such as pinholes, can dramatically limit the efficacy of the separations. Testing membranes for these defects generally requires their installation and operational testing at considerable expense. This research focuses on the development of a rapid colorimetric test based on mono-dispersed fluorescent particles that would identify defects in membranes by molecular sieving. Cadmium selenide quantum dots, dye modified polyamidoamine dendrimers and silica (Stober) spheres were chosen as the test probes because they can be prepared with very narrow distributions in size. This approach allows defects to be identified and their size estimated based on the passage of the fluorescent probes across the membranes. The fluorescence of the probes permits their detection spectrophotometrically. Membranes with well defined porosity provide the means to relate probe size and sieving to the size and topology of defects.