The Pittsburgh activated carbons (Calgon Co.) were outgassed at 1, 000°C for 2 hours and then reduced with H₂ at 1, 000°C or oxidized with O₂ at 100 and 300°C to prepare three activated carbons with different surface oxygen content. The prepared activated carbons have almost the same specific surface areas, pore volumes and pore size distributions. On these activated carbons, adsorption isotherms of water, ethanol and benzene were obtained. The effect of oxygen present on the surface of the activated carbons upon the adsorbability of these molecules was discussed. The amount of both water and ethanol adsorbed increased with increasing surface oxygen content of the activated carbon. On the other hand, that of benzene was unchanged. It is thought that the increases of the adsorbability of water and ethanol with increasing surface oxygen content are due to the interactions between the surface oxygen sites and the polar groups of these molecules. The reason for the constant adsorbability of benzene on these carbons may be that the adsorption of the benzene molecule takes place, it is thought mainly on hexagonal planes of the activated carbon crystallites, and that benzene is a non-polar molecule and so may not interact with surface oxygen sites.