Cesium salts of Cs2SO4, Cs2CO3 and CsHSO4 and phosphotungstic acid (H3PW12O40 center dot 6H(2)O:WPA center dot 6H(2)O) were mechanically milled to synthesize chemically durable WPA composites. The X-ray diffraction patterns of composites suggested the formation of partially substituted CsxH3-xPW12O40 composites after milling at 720 rpm for 10 min. Proton conductivities under both humidified and dry conditions of the resultant composites markedly improved. The proton conductivities strongly depended on the type of cesium salt. The hydrogen bonding distance L in the frame structures of the composites was estimated from the H-1-isotropic chemical shift. The proton conductivities in dry conditions of WPA center dot 6H(2)O and the composites were strongly represented to L. (c) 2008 Elsevier B.V. All rights reserved.