The microstructure (specific surface area and pore size distribution), amount of hydrated water, and proton conductivity were examined for hydrated tin dioxide (SnO2.nH(2)O) and hydrated zirconia (ZrO2.nH(2)O), and their relationships were investigated. Both hydrates had many micropores with a pore radius below I rim. The average pore size of SnO2.nH(2)O was smaller than that of ZrO2.nH(2)O. The amount of hydrated water n was 0.7-1.2 for SnO2-nH(2)O and 1.0-1.7 for ZrO2.nH(2)O under a relative humidity (RH) of similar to0 to 95% at 150 degreesC. Proton conductivity was approximately 10(-2) S cm(-1) under 95% RH at 150 degreesC for both hydrates. The conductivity decreased with decreasing RH, but SnO2.nH(2)O maintained a higher conductivity than that of ZrO2.nH(2)O under low RH. The high conductivity of SnO2.nH(2)O at high RH despite low hydrated water content was assumed to be due to the high electronegativity of Sri and resulting high concentration of dissociated protons. The hydrated water in the small micropores of SnO2.nH(2)O will be maintained even under low humidity (due to the high adsorption energy in the small micropore), and this was considered to result in the small dependence of conductivity on RH for SnO2.nH(2)O.