Hydrous Ta2O5 . nH(2)O has been synthesized by a solution-based procedure and incorporated into Nafion to obtain Nafion-Ta2O5 . nH(2)O composite membranes. The composite membranes have been evaluated in proton exchange membrane fuel cells (PEMFCs) at different temperatures in the presence and absence of CO impurities in H-2 fuel, and the data have been compared with those obtained with unmodified recast Nafion membrane. Membrane-electrode assemblies (MEAs) fabricated with the composite membranes and electrodes consisting of small amounts of hydrous Ta2O5 . nH(2)O show better electrochemical performance at higher temperatures (100- 110 degreesC) compared to the native recast Nafion membrane due to their ability to retain water at higher temperatures. For example, the MEAs consisting of Ta2O5 . nH(2)O and 50 mum thick membranes show high power densities of similar to800 and 650 mW/cm(2) at 100 degreesC, respectively, with pure H-2 and H-2-CO (150 ppm CO) fuels. (C) 2004 The Electrochemical Society.