The composition effect of polyacryl onitrile-block-poly(m ethyl acrylate) (P(AN-b-MA)) block copolymer on the swelling behavior and the pervaporation performance of the resultant membranes were investigated for separating benzene/cyclohexane mixtures. It was found that the degree of swelling (DS) of membranes increased with increasing the MA content in P(AN-b-MA) block copolymer and the benzene content in feed at 30 degrees C. The results also showed that the benzene flux (J(benzene)) of P(AN-b-MA) membranes increased and separation factor (alpha) decreased with increasing MA content in the membrane, the benzene content in feed, and the feed temperature. Further, the dependence of the swelling behavior and pervaporation performance on the MA content in the membranes revealed a discontinuity phenomenon. The DS and the benzene flux increased dramatically when the MA content in the membrane exceeded 40 mol%. Also, P(AN-b-MA) block copolymer membranes with less or more than 40 mol% MA content revealed different surface morphologies from both scanning electron microscopy (SEM) images and atomic force microscopy (AFM). All these might be explained by the transition of MA segment from a dispersion phase to a continuous phase with increasing MA content in the P(AN-b-MA) block copolymer membranes. (C) 2008 Elsevier B.V All rights reserved.