Membranes prepared from carboxylated polysulfone permeated benzene preferentially from benzene- cyclohexane mixtures by pervaporation. Hydrophilic polysulfones containing carboxyl groups with degrees of substitution DSI ranging from 0.46 to 1.90 groups per repeating unit were selected for this study. The membrane having a carboxyl group DS of 0.88 showed the best trade-off between flux and permselectivity. The permselectivity towards benzene was 234 at the weight fraction of benzene in feed of 0.1. From pervaporation and sorption experiments, it was evident that permselectivity toward benzene was due to solubility selectivity.