Nafion/silicon oxide (SiO2)/phosphotungstic acid (PWA) and Nafion/silicon oxide composite membranes were studied for the H-2/O-2 proton exchange membrane fuel cells (PEMFCs) operated above 100degreesC. The composite membranes were prepared by the recasting procedure, using Nafion solution mixed with SiO2 and PWA/SiO2 Mixtures. The physico-chemical properties of these recast composite membranes were studied by means of scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis, thermogravimetry-DTA (TG-DTA) and Fourier transform infrared (FTIR) spectroscopy. These results showed that SiO2 and PWA are compatible with the Nation membrane. Furthermore, the incorporation of the SiO2 and PWA into the Nation membrane could increase the crystallinity of the Nafion recast membrane and also improve the initial degradation temperature of the Nation membrane. It was found that the composite membrane showed a higher uptake of water compared with the Nafion recast membrane. The proton conductivity of the composite membranes appeared to be similar to that of the native Nafion membrane at high temperatures and at 100% relative humidity (RH), however, it was much higher at low RH. When the composite membranes viz. Nation/SiO2/PWA and Nafion/SiO2 were employed as an electrolyte in H-2/O-2 PEMFC, the higher current density values (540 and 320 mA/cm(2) at 0.4 V, respectively) were obtained than that of the Nafion 115 membrane (95 mA/cm(2)), under the operating condition of 110degreesC and at the humidified temperature of 100degreesC. (C) 2003 Elsevier B.V. All rights reserved.