CsHSO 4 and dodecatungstophosphoric acid (H 3 PW 12 O 40 ·6H 2 O:WPA·6H 2 O) were mechanically milled to synthesize xCsHSO 4 ·(100−x)WPA (mol%) composites. 1 H NMR measurements suggested that a hydrogen bond was newly developed between WPA and CsHSO 4 , and proton transfer at CsHSO 4 –WPA composite region was considered to be faster than that at pure CsHSO 4 or WPA. Proton conductivity of the 90CsHSO 4 ·10WPA (mol%) composite at 100 °C showed much higher of 3.3×10 −3 S/cm than that of pure CsHSO 4 (4×10 −7 S/cm) or that of WPA(1×10 −7 S/cm). The –O(H)···O hydrogen bonding distance was estimated from the 1 H-isotropic chemical shift of 1 H MAS NMR spectra. The proton conductivities in dry atmosphere of CsHSO 4 ·WPA composites were strongly correlated with the O(H)···O bond distance L.