For dehydrating a water/ethanol mixture by pervaporation, a sulfonated polysulfone membrane was prepared. The separation performance of water and ethanol are shown to strongly depend on the degree of substitution of polysulfone membrane. The degree of substitution increased with increasing chlorosulfonic acid in the casting solution, and the substitution reaction was achieved within 2 h. The water permeation rate and separation factor increased with increasing substitution of polysulfone membrane up to a substitution of 2.0. The effect of sulfonation on separation performance was due to the improvement of hydrophilicity of sulfonated membrane. It was found that the solubility of water/ethanol in the membrane was not the dominant factor for separation but it was rather the diffusion difference in the membrane. The diffusion difference between permeate through sulfonated membrane was the dominant factor for separating the water/ethanol mixture. The high performance of pervaporation membrane can be prepared by sulfonated polysulfone.