Poly(vinylidene chloride-co-vinyl chloride)-graft-poly(butyl methacrylate), which was prepared by a radical graft polymerization of butyl methacrylate onto poly(vinylidene chloride-co-vinyl chloride) and poly(vinylidene chloride-co-vinyl chloride), itself was found to be suitable for the membrane materials for the selective separation of benzene from benzene/cyclohexane mixtures in the whole feed composition range. The permeation rate was enhanced by the introduction of poly(butyl methacrylate) onto poly(vinylidene chloride-co-vinyl chloride) membrane.