An anhydrous proton conductor, 10 mol % In3+-doped SnP2O7 (Sn0.9In0.1P2O7), was composed by 1,8-bis(triethoxysilyl)octane (TES-Oct) and 3-(trihydroxysilyl)-1-propanesulfonic acid ((THS)Pro-SO3H) and was characterized by structural and electrochemical analysis. The composite membrane with 90 wt % Sn0.9In0.1P2O7 showed high proton conductivities of 0.04 S cm(-1) or more between 150 and 200 degrees C in unhumidified air. The packing of the Sn0.9In0.1P2O7 particles in the matrix was relatively uniform, with no formation of pinholes observed. Fuel cell tests verified that the open-circuit voltage was maintained at a constant value of similar to 970 mV regardless of the electrolyte thickness (60 - 200 mu m), while the Ohmic resistance was decreased to 0.24 Omega cm(2) by reducing the electrolyte thickness to 60 mu m. The peak power densities achieved with unhumidified H-2 and air were 109 mW cm(-2) at 100 degrees C, 149 mW cm(-2) at 150 degrees C, and 187 mW cm(-2) at 200 degrees C. Furthermore, fuel cell performance was improved by hot-pressing an intermediate layer consisting of Sn0.9In0.1P2O7, Pt/C, TES-Oct, and (THS) Pro-SO3H between the electrolyte and cathode. (c) 2006 The Electrochemical Society.