Atomic force microscopy (AFM) was used to investigate the surface of polysulfone (PSf) membranes. The AFM method provides information on both size and shape of pores or cavities on the surface as well as the roughness of the skin. The pore sizes obtained from AFM observation were found to be more accurate than those obtained from scanning electron microscopy (SEM) since the potential of altering the pore structure of the membrane during sample preparation was eliminated. It was observed that two different modes of phase separation existed during the formation of PSf membrane when the coagulation conditions were varied.