The effect of carbon modifications and oxygen surface groups on the hydrophobicity of activated carbons is described. The hydrophobicity was determined by competitive adsorption of water and toluene vapor. From the breakthrough curves the hydrophobicity index (HI) was determined, which is defined as the adsorbed amount of toluene, relative to the adsorbed amount of water. The adsorption of toluene is only determined by the available pore volume. The water adsorption, however, increases with both the porosity and the amount of acidic surface sites; This is in line with H-2 solid state NMR experiments, which showed that the presence of toluene decreases the mobility of water,:either due to a stronger interaction with the surface sites or to an enhanced adsorption in the smallest pores. Ion exchange of the carbons with alkali metals favors the water adsorption. The pore size, compared to the ion size within the series alkali-metal ions, resulted in an optimum hydrophilicity upon Na+ exchange.