Soluble sulfonated polyimides (PI) were heterogeneously synthesized by reacting soluble PIs with sulfur trioxide and dichloromethane solvent. The sulfonated PIs were soluble in polar solvents like N-methyl-2-pyrrolidone (NMP) and N,N'-dimethylformamide. The sulfonated PIs were characterized by Fourier transform infrared spectroscopy, proton nuclear magnetic resonance spectroscopy, differential scanning calorimetry, thermogravimetric analysis, triangular phase diagram, and back titration. Asymmetric nanofiltration membranes were prepared by the phase inversion method from the casting solution containing NMP and diethyleneglycol dimethylether (DGDE). Introducing DGDE as an additive in the casting solution decreased pore size. The rejection rates of organic molecules and salts were investigated. The observed salt rejection of the membrane was of the order R(Na2SO4) > R(NaCl) > R(CaCl2). The membrane morphology was investigated by scanning electron microscopy. (C) 2003 Wiley Periodicals, Inc.