For dehydrating water/ethanol mixture by pervaporation, a lithiated polysulfone membrane was prepared. The separation performance of water and ethanol strongly depend on the degree of lithiation of polysulfone (PSF) membrane. The water permeation rate decreased and separation factor increased with increasing the degree of lithiation of polysulfone membrane upto 0.75. Beyond the degree of substitution 0.75, the permeation rate increased and separation factor decreased with increasing the substitution. The effect of lithiation on separation performance was due to the improvement of diffusion selectivity lithiated membrane. It was found that the diffusion selectivity was the dominant contribution to overall permselectivity. The diffusion difference between permeates through lithiated membrane was the dominant factor for separating water/ethanol mixture.