The sulfonation of polymers and the pervaporation behavior of asymmetric polymeric membranes are described in this study. We confirmed that the sulfonic acid group was successively introduced into the polymer chain by FTIR and H-1-NMR characterization. The phase diagram indicated that the modified polymer could have water tolerance, implying that the polymer-solvent miscibility was lower. The hyperthin skin layer, suitable for pervaporation, depended on the NMP-DGDE composition, which was important for the fabrication of an asymmetric membrane with high water permeance. The asymmetric membrane exhibited water selectivities equal to or slightly lower than those determined for the dense film. The hyperthin skin layer, which had hardly any defects, was possibly formed by the phase-inversion method. It is obvious that these membranes can be used for the industrial pervaporation process. The permeability and the selectivity of the water-butanol mixture for the asymmetric membranes at 50 degrees C were measured. The permeation rates for water-butanol in the asymmetric membrane were about 80 times greater than those of film, and the separation factor was slightly lower. The NMP-DGDE solvent system might be suitable for a high permeation rate. (C) 2000 John Wiley & Sons, Inc.