Grafting of styrene on to Nation membranes is carried out by impregnation and radical polymerization in supercritical carbon dioxide (scCO(2)) as a solvent and swelling agent. Styrene monomer and the initiator 2,2'-azoisobutyronitrile (AIBN) are first impregnated into Nation membranes using scCO(2) at 38degreesC and 25 MPa. After releasing CO2 the polymerization is carried out at 80degreesC and 10 MPa. The Nafion-grafted-polystyrene (N-g-ps) is sulfonated in concentrated sulfuric acid (98% H2SO4). The grafted membranes are characterized by measuring their ion-exchange capacities, ion conductivity and methanol permeation. The morphology and structure of these membranes are observed with Fourier transform infrared spectroscopy and scanning electron microscopy. The Nafion-grafted-polystyrene sulfonic acid (N-g-pssa) exhibits higher ion conductivity and lower methanol permeability than that of Nafion 115. The N-g-pssa membranes are tested as electrolytes in a direct methanol fuel cell. (C) 2004 Elsevier B.V. All rights reserved.