A new composite membrane is fabricated by incorporating tungsten trioxide hydrate into Nafion to be employed as a candidate electrolyte for proton exchange membrane fuel cells (PEMFCs) operated above 100 degrees C. Thermal behavior and proton conductivity of the composite membrane are studied by means of thermogravimetric/differential thermal analyis (TG/DTA) and AC impedance measurements, respectively. These results demonstrate that the thermal stability of the composite membrane has no appreciable change when compared with the native Nafion membrane. The proton conductivity of the composite membrane is found to be better than that of the native Nafion membrane at high temperature and lower relative humidity. When the composite membrane is used as an electrolyte in H-2/O-2 PEMFC under the operating conditions of 110 degrees C, 1.36 atm gas pressure, and 70% relative humidity, the observed current density value at 0.4V is 1.5 times higher than that of the cell employing native Nafion membrane as an electrolyte.