For dehydrating a water-ethanol mixture by pervaporation, a polysulfone-And poly(ethylene glycol)(PSF-PEG) membrane was prepared. The separation performance of water and ethanol was found to strongly depend on the diffusion selectivity of permeates. On the other hand, the solubility selectivity of water to ethanol showed only minor change with an increasing PEG composition of the PSF-PEG membrane. This study found that the PEG content in a FSF-PEG membrane showed mobility enhancement of pervaporation properties and that the diffusion difference of Permeates increased with increasing PEG content The effect of PEG content on separation performance was a result of the improvement of the permeate diffusion properties of the PSF/PEG membrane. The diffusion difference in the membrane, not the solubility of water-ethanol in the membrane, was the dominant factor for the separation. Suitable PEG content in PSF/PEG membranes can prepare a high-performance pervaporation membrane. (C) 2003 Wiley Periodicals, Inc.