The cloud point curves for a ternary system of PSf/NMP/water were determined by a titration method at 15 degrees C, 30 degrees C, 45 degrees C and 60 degrees C. A small amount of water (3-10 wt% water) was needed to achieve liquid-liquid phase separation and the temperature effect was not significant. The vitrification composition for the ternary system of PSf/NMP/water was determined by DSC measurements for various compositions. Cross-sectional membrane morphologies were examined by SEM (Scanning Electron Microscope) and surface morphologies by AFM (Atomic Force Microscope) varying the parameters including polymer concentration and bath compositions. It was observed that there were two different modes of phase separation during the formation of polysulfone membrane when the coagulation conditions were varied.