Composite membranes based on phosphotungstic acid (PWA) adsorbed on silica (SiO2) and polybenzimidazole (PBI) have been prepared and their physico-chemical properties have been studied. The membranes with high tensile strength and thickness of less than 30 mu m can be cast. They are chemically stable in boiling water and thermally stable in air up to 400 degrees C. Proton conductivity is influenced by the temperature (range: 30-100 degrees C), relative humidity and PWA loading in the membrane. Maximum conductivity of 3.0 x 10(-3) S/cm is obtained at 100% relative humidity and 100 degrees C with membrane containing 60 wt.% PWA/SiO2 in PBI. Conductivity measurements performed at higher temperatures, in the range from 90 degrees C to 150 degrees C, give almost stable values of 1.4-1.5 X 10(-3) S/cm at 100% relative humidity.