A class of new proton-electrolyte membranes (PEM) based on inorganic-organic copolymers were synthesized from 3-glycidoxypropyltrimethoxysilane (GPTS), sulfonated phenyltriethoxysilane (SPS), tetraethoxysilane (TEOS) and H3PO4. Their thermal stability, microstructure, and proton conductivity were investigated under the conditions for PEM fuel cell operation. TGA-DSC analysis indicated that these membranes are thermally stable up to 180degreesC. Scanning electron microscope (SEM) micrographs show that the membranes are dense. A proton conductivity of 1.6 x 10(-3) S/cm was observed at 100degreesC in a dry atmosphere for a sample with 0.5 mol GPTS and I mol H3PO4 in I mol Si, representing the highest proton conductivity in anhydrous state among PEMs ever reported. In an environment with 15% relative humidity (RH), the proton conductivity increased to 3.6 x 10(-2) S/cm at 120degreesC. The proton conductivity increases with H3PO4 contents and relative humidity. The hybrid inorganic-organic materials can be readily fabricated in membrane form with thickness as thin as 20 mum on porous electrodes; they have great potential to be used as the electrolytes for high-temperature PEM fuel cells. (C) 2003 Elsevier Ltd. All rights reserved.