The oxidative stability and proton conductivity of a sulfonated polystyrene/poly(vinyl chloride) (SPS/PVC) composite membrane were simultaneously enhanced through the proper copolymerization of styrene (St) with alpha-methylstyrene (alpha MS), acrylonitrile (AN), and divinylbenzene (DvB). These monomers were readily impregnated and polymerized within a nonporous PVC film, thereby forming a semi-IPN structure. The AN increased both the molecular weight of copolymer and swelling of the PVC film in the monomer solution composed of St, aMS, AN, and DVB. The molecular weight and weight gain ratio affected the degradation mechanism and membrane properties during the oxidative degradation of a crosslinked copolymer reinforced with PVC film. The low AN content (5-10%) limited the enhancement of the oxidative stability of the copolymer due to its low molecular weight. And though AN effectively increased the copolymer molecular weight, excessive AN content (30%) slightly deteriorated both the proton conductivity and oxidative stability due to the high water uptake. In this study, a proper AN content (20%) concurrently improved both the proton conductivity (0.164 S/cm) and oxidative stability compared to the SPS/PVC composite membrane. (C) 2010 Elsevier B.V. All rights reserved.