A high conductivity composite proton-exchange membrane Cs2.5H0.5PMo12O40 (CSPOM)/polybenzimidazole (PBI) for use in hydrogen proton-exchange fuel cells has been prepared. The CsPOM composite membrane is insoluble in water. The composite membrane doped with H3PO4 showed high-proton conductivity (>0.15 S cm(-1)) and good thermal stability. P-31 NMR analysis has suggested the formation of a chemical bond between the CsPOM and PBI in the composite membrane. The performance of the membrane in a high-temperature proton-exchange membrane fuel cell (PEMFC) fueled with hydrogen was better than that with a phosphoric acid-doped PBI membrane under the same conditions and at temperatures greater than 150 C. The CsPOM/PBI composite would appear to be a promising material for high-temperature PEMFC applications. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.