Nafion membranes suffer high resistance due to dehydration when they are used at elevated temperatures (>100degreesC) and atmospheric pressures (Nafion 1135 > 0.6 Ohm cm(2) at 120degreesC and 31% relative humidity). Thinner composite membranes containing Nafion and zirconium hydrogen phosphate, Zr(HPO4)(2), were prepared and showed lower resistance (<0.3 Ohm cm(2)) at these conditions. Thermogravimetric analysis showed that the composite membrane was stable below 200degreesC in oxygen. Hydrogen crossover of a 18 mum thick composite membrane was only 2 mA/cm(2) at 120degreesC. When the temperature of a cell using this membrane was raised from 80degreesC (100% relative humidity) to 120degreesC (31% relative humidity), the main performance loss at the elevated temperature was from low protonic conductivity and low oxygen permeability in the cathode catalyst layer and dehydration of the membrane. (C) 2004 The Electrochemical Society.