The dielectric and electrical behaviors of a Nafion(TM) membrane, Nafion(TM)/7SiO(2)-2P(2)O(5)-ZrO2 (SPZ), Nafion(TM)/SiO2 + H3PO4 (iHPO), Nafion(TM)/SiO2 +H3PW12O40 (iHPW) and Nafion(TM)/ZrP composites were investigated using water desorption measurements and ac impedance spectroscopy as a function of time, relative humidity and temperature. The composite membranes except Nafion(TM)/ZrP exhibited only marginally higher conductivity than Nafion(TM) at high temperature. Unlike the other membranes, the Nafion(TM)/ZrP membrane showed reduced proton mobility with a higher activation energy. The results suggest that improvements in fuel cell performance, previously observed in Nafion(TM)/SPZ and Nafion(TM)/ZrP composite membranes, are not due simply to increased hydrophilic properties, but that there is an interplay between structure and proton mobility. (C) 2003 Elsevier B.V. All rights reserved.