The effect of the O3 treatment of the hydrogenated graphite on the adsorbability of H2O was investigated by measuring the adsorption isotherm of H2O and the amount of gas evolved on heating the sample. After the O3 treatment of the graphite, the amount of surface oxides became much larger, the monolayer capacity, V(m), of H2O increased drastically, and the isosteric heat of adsorption, q(st), of H2O became much higher than the heat of liquefaction of H2O. Namely, the O3 treatment made the surface of the graphite very hydrophilic. The further heat treatments of the O3-treated graphite at 500 and 1000-degrees-C made the amount of surface oxides and the V(m) value of H2O gradually smaller. At the same time, the q(st) values were depressed far below the heat of liquefaction of H2O, and especially the qt curve on the 1000-degrees-C treated sample revealed a deep minimum. Such a change in the adsorbability of H2O on graphite was interpreted in terms of the species and the amount of surface oxides.