Proton conductivity of hydrated tin dioxide (SnO2.nH(2)O) was investigated at 100-150degreesC under water vapor pressure up to 0.5MPa, and was compared with that of hydrated zirconia (ZrO2.nH(2)O). High proton conductivities above 10(-2) S cm(-1) were observed in both hydrates at 150 degreesC under high water vapor pressures. The conductivity decreased with a decrease in water vapor pressure at 130-150 degreesC, but the decrease of conductivity in SnO2.nH(2)O was smaller than that in ZrO2.nH(2)O. The SnO2.nH(2)O showed a good recovering property of conductivity after drying at 150 degreesC, which was not observed for ZrO2.nH(2)O, suggesting the SnO2.nH(2)O to be suitable as proton-conductive electrolyte for use at intermediate temperature. Films of SnO2.nH(2)O/polymer composites were prepared, and a high conductivity was observed in SnO2.nH(2)O/R-epoxy resin.