To develop highly efficient adsorbents for water vapor which may be applicable to adsorption heat pumps (AHPs), an attempt was made to design and manufacture novel functional active carbons by hydrophilic treatment. In the present work, five different active carbons were subjected to oxidation for 2 to 4 h by use of HCl and HNO3 in the concentration range of 0.5-1.0 mol/dm(3). For the evaluation of water vapor adsorption characteristics of the treated active carbon, water vapor isotherm at 298 K and the amount of functional groups (Fs) of the oxidized active carbon were measured. Based on the isotherm obtained, the contact angle (theta) between pore surface and water vapor was estimated. All isotherms of the active carbons treated were found to be correlated with the Kelvin equation, which is based on the assumption that capillary condensation takes place uniformly on each adsorption site. It was also found that the isotherm of the oxidized active carbon shifted to lower relative vapor pressure (phi) than with that of non-treated active carbon. The logarithm of phi(0.5), which corresponds to the relative pressure at q/q(phi=1)=0.5 (q; amount of adsorbed), decreased linearly with increasing of cos theta. The Fs value also decreased with decreasing of cos theta. These results confirm that the amount of hydrophilic functional groups can be an indicator the performance of active carbon for water vapor adsorption. Furthermore, the amounts of water vapor adsorbed on the oxidized active carbon were estimated to be 1.2 and 1.9 times higher than that of silica gel in the relative water vapor pressure range for the operation of an AHP of the closed type and desiccant type, respectively.