An activated carbon Carbochem (TM) - PS230 was modified by chemical and thermal treatment in flow of H-2 in order to evaluate the influence of the activated carbon chemical surface in the adsorption of the monohydroxylated phenols. The solid-solution interaction was determined by analyzing the adsorption isotherms at 298 K at pH 7, 9 and 11 during 48 h. The adsorption capacity of activated carbons increases when the pH solution decreases. The amount adsorbed increases in the reduced carbon at the maximum adsorption pH and decreases in the oxidized carbon. in the sample of granulated activated carbon, CAG, the rnonohydroxylated phenols adsorption capacity diminishes in the following order catechol > hydroquinone > resorcinol, at the three pH values. The experimental data are evaluated with Freundlich's and Langmuir's models. The immersion enthalpies are determined and increase with the retained amount, ranging between 21.5 and 45.7 J g(-1). In addition, the immersion enthalpies show more interaction with the reduced activated carbon that has lower total acidity contents. (C) 2009 Elsevier B.V. All rights reserved.