The characteristics and mechanism of proton conduction in organic/inorganic composite membranes composed of sulfonated poly(ether sulfone) (SPES) and layered tin phosphate hydrates Sn(HPO4)(2) center dot nH(2)O (SnP) were examined. SPES/SnP membranes were prepared by mixing 0-50 vol% of SnP with SPES solution and casting the mixture. Fourier-transform infrared, Raman, and solid-state H-1 NMR spectroscopy and thermogravimetric analysis showed that hydrogen-bond networks are formed between SPES and SnP in the composite membranes. At intermediate temperatures up to 150 degrees C, the membranes showed good proton-conducting properties, high chemical stability (low water solubility), and high thermal stability as a result of the formation of hydrogen bonds between SPES and SnP. The membrane containing 35 vol% SnP showed the highest conductivity of 8.1 x 10(-2) S cm(-1) at 130 degrees C under saturated water vapor pressure. The membrane containing 25 vol% SnP showed a conductivity of 5.3 x 10(-2) S cm(-1) at 130 degrees C under saturated water vapor pressure, despite its low water uptake. This membrane retained a high conductivity of 2.6 x 10(-3) S cm(-1) at 130 degrees C at a low relative humidity of 40%, indicating that SPES/SnP membrane is a promising proton conductor at intermediate temperatures. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.006202jes] All rights reserved.