The vitrification line for the ternary system of polysulfone (PSf)/N-methyl 2-pyrrolidinone (NMP)/water was determined by differential scanning calorimeter (DSC) measurements with varying compositions. Pure PSf showed both alpha- and beta-transition temperatures (T-g,T-alpha = 187.5 degrees C, T-g,T-beta = -21.4 degrees C). The T-g,T-alpha of PSf decreased with increasing solvent concentration. The T-g,T-alpha of PSf decreased linearly with the addition of NMP in the concentration range of 70-90 wt % polymer. The vitrification line was indicated in the phase diagram for the ternary system of PSf/NMP/water at 15 and 60 degrees C. As the temperature is increased, a high polymer concentration was needed to reach the vitrification condition. The vitrification composition of the polymer in the binary system of PSf and NMP was 72.0 wt % at 15 degrees C and 79.8 wt % at 60 degrees C. We also found that the slope of the vitrification line changed with the temperature and that a small amount of water (10-20 wt %) can induce the vitrification of the polymer solution in the PSf/NMP/water system at 15 degrees C. (C) 1999 John Wiley & Sons, Inc.