The increase of the operation temperature in proton exchange membrane fuel cell (PEMFC) above 100 degreesC is a great concern for the application of this type of cells in electric vehicles. Hybrid organic-inorganic membranes with nanosized interfaces can combine the main properties to meet this objective. Poly (styrene-co-methacrylate)-silica covalent hybrid membranes have been synthesised by copolymerization of monomers (styrene (STY) and 2-hydroxyethyl methacrylate (HEMA)), with formation of covalent bonds between hydroxyl group from the latter and pre-hydrolyzed tetraethoxysilane. Tungstophosphoric acid hydrate was incorporated to endow the membranes of proton conductivity. The optimal composition and synthesis conditions to promote organic polymerization and sol-gel condensation avoiding phase separation are important issues of this work. The structural analysis shows homogeneous membranes without phase separation. The combination of water uptake and water retention properties provided by SiO2 and tungstophosphoric acid leads to high proton conductivity (maximum values around 1 S/cm) at 120 degreesC. (C) 2004 Elsevier B.V. All rights reserved.