Characterization of porosity in porous membranes or defects in non-porous membranes can be difficult do except by the time-consuming method of measuring the permeation of gases or chemicals. Ultrafiltration membranes are often characterized by their ability to prevent bacteria or viruses from passing or sieving (size exclusion). However, these evaluations require culturing and characterizing microorganisms. In this research we are examining the use of fluorescent particles to rapidly characterize the size of pores or defects in membranes by size exclusion. Silica stober spheres were prepared in batches ranging from 100-330 nm in diameter, then modified with Dansyl propyltriethoxysilane to make the particles fluorescent. These particles can be made with nearly monomodal size distributions, making them ideal for characterizing porosity by sieving. Suspensions of the fluorescent particles were placed on membranes with different sized pores and the permeate was examined for fluorescence that would indicate pores were at least as large as the particles. To date, we have been able to demonstrate that the fluorescent particles can be used to characterize porous membranes.