The surface of activated carbon fiber (ACF) was oxidized by a H2O2 solution or was reduced by H-2 gas and the change in the water adsorption isotherm as a result of such surface treatments was examined at 303 K. The surface oxidation by H2O2 increased the amount of adsobed water in the low-pressure region, while the reduction by H-2 decreased the low-pressure adsorption of water. The surface oxidation also increased slightly the intensity of the XPS C-1s tail in the high-energy region. The XPS ratio spectrum, using the spectrum of the reduced ACF as the standard, gave an explicit peak due to the presence of surface functional grops. The ratio peak area had a linear relationship with the amount of adsobed water in the low-pressure region.