Novel thin film composite electrolyte membranes, prepared by dispersion of nano-sized SiO2 particles in the solid acid compound KH(PO3H), can be operated under both oxidizing and reducing conditions. Long-term stable proton conductivity is observed at similar to 140 degrees C, i.e., slightly above the superprotonic phase transition temperature of KH(PO3H), under conditions of relatively low humidification (pH(2)O approximate to 0.02 atm). (C) 2009 Elsevier B.V. All rights reserved.