A limitation of Nation membranes in proton-exchange membrane fuel cells (PEMFCs) is their poor water retention above 100 degrees C. The incorporation of SiO2-P2O5-ZrO2 gels into Nation membranes can improve their thermal stability. Therefore, composite membranes were prepared either by infiltration of Nation membranes with SiO2-P2O5-ZrO2 sol or by recasting a film using Nation solution and SiO2-P2O5-ZrO2 sol. The membranes were characterized by thermal analysis, microstructural analysis, functional testing in a fuel cell, and electrochemical impedance spectroscopy. The incorporation of the SiO2-P2O5-ZrO2 gel resulted in improved electrochemical behavior at 130 degrees C, with current density-cell potential values as good as those obtained at 80 degrees C with unmodified Nation membranes. (c) 2005 The Electrochemical Society. [DOI: 10. 1149/1.1852443] All rights reserved.