Polypropylene (PP)-g-sulfonated polystyrene (SPS) composite membranes were prepared by grafting polystyrene (PS) on microporous polypropylene membranes via plasma-induced polymerization. Grafting of polystyrene was established not only inside the pores but also on the surface of PP membranes, followed by the sulfonation reaction. The chemical and physical structure of PS-g-PP membranes was investigated using FTIR and SEM. The thickness and weight of the composite membrane increased with increasing grafting time. Ion exchange capacity (IEC), ion conductivity, and methanol permeability coefficient were measured and analyzed according to grafting reaction and sulfonation time. While both the ion conductivity and methanol permeability coefficient increased with grafting amount, the characteristic factor was comparable to that of Nafion((R)). (C) 2003 Elsevier B.V. All rights reserved.