Atomic force and scanning tunneling microscopes have been used to investigate the surface of polymeric ultrafiltration and microfiltration membranes. The atomic force microscope has been shown to resolve the features of the selective skin layer of the membranes providing information on both size and shape of pores as well as surface roughness of the skin. When applied to the study of ultrafiltration and microfiltration membrane for the determination of pore size characteristics, the scanning tunneling microscope has been shown to give similar results with those obtained using polyethylene glycols transport measurements. The microscopic techniques have discerned pore diameters in the range of 11 to 114 nm for 40 to 200 kDa MWCO SPS ultrafiltration membranes and confirmed a mean pore diameter of 96 nm for the 0.1 mu m PVDF microfiltration membrane.