Powders of hemihydrate gypsum (CaSO(4)(.)0.5H(2)O) are set and hardened at room temperature by mixing with a phosphoric acid solution to be formed into the dihydrate gypsum (CaSO(4)(.)2H(2)O) membrane. These gypsum-derived membranes exhibit superprotonic conductivities of approximately 10(-2) S cm(-1) in the entire temperature range from -10 to 200 degrees C. A H-2/air configuration cell using 0.4 mm thick electrolyte membranes is operated in nonhumidified conditions at 20 degrees C (max. power density similar to 44 mW cm(-2)) to 80 degrees C (max. power density similar to 65 mW cm(-2)). (c) 2007 The Electrochemical Society.