The cloud point curves for polysulfone (PSf)/solvent/water systems were determined by a titration method. A small amount of water was needed to induce liquid-liquid demixing and the temperature effect was small. From numerical calculations, it was found that the binary interaction parameters for the PSf/solvent/water system enlarges the homogeneous region in the phase diagram with a smaller nonsolvent-polymer interaction parameter chi(13), a greater nonsolvent-solvent interaction parameter chi(12), and a smaller solvent-polymer interaction parameter chi(23) and the effect of polymer molecular weight was negligible except in the range of low molecular weight. The phase diagrams, calculated with constant chi 12 that was chosen from the concentration-dependent interaction parameter g(12) value of the concentration range, were similar to the results obtained with g(12). The slope of the tie lines indicated that demixing of the ternary system occurred at relatively similar nonsolvent concentration in both phases. A value of 2.7 for the water-PSf interaction parameter was obtained by fitting the experimental cloud point curve with the calculated binodal lines.