The proton conductivity, and stability Of Cs2HPW12O40 above 100 degrees C were investigated under various humidified conditions. The electrolyte showed the maximum conductivity at 100 degrees C, and the conductivity continuously decreased with an increase in temperature, even under highly humidified conditions. In wet conditions, the samples were mechanically stable and showed the constant conductivity with no degradation for 20 h. These results suggested that the water absorbed into the electrolyte played an important role on the proton conduction mechanism. The fuel cell based on Cs2HPW12O40 electrolyte (thickness; 1.25 mm) was fabricated and the power generation in H-2/O-2 fuel cells using the Cs2HPW12O40 electrolyte was firstly demonstrated at 120 degrees C. (c) 2005 Elsevier B.V. All rights reserved.