2-Hydroxyethyl methacrylate (HEMA) (branch)-methyl acrylate (MA) (backbone) comb-type graft copolymers were synthesized by grafting an amino-semitelechelic oligo (HEMA) to a methacrylate (GMA) random copolymer. From electron microscope observation, it was found that these graft polymeric membranes had microphase separated structures. Pervaporation of a benzene-cyclohexane mixture through these membranes was carried out in order to study the effect of microphase separated structures on selective permeability. Benzene was found to preferentially permeate through the membranes. Moreover, the pervaporation rate of benzene increased with increasing MA mole fraction in the graft copolymers. It was found that the continuous phases ofpoly(MA) domains functioned as permeating pathways for benzene and that the selectivity of the graft polymeric membranes occurred in the process of dissolution of the solvent into the membranes. The microphase separated structure was found to effectively enhance the selectivity as a result of its suppressing the plasticizing effect of benzene. KEY WORDS Pervaporation I Benzene-Cyclohexane Azeotropic Mixture I Membrane I Graft Copolymer I 2-Hydroxyethyl Methacrylate I Methyl Acrylate I Microphase Separated Structure I Plasticizing Effect I Separation I