Adsorption of valeric acid was studied on two activated carbons of different origins, wood and bituminous coal. The effect of oxidation on the adsorption uptake was investigated. Both initial samples were oxidized with nitric acid, which caused the introduction of a significant number of oxygen-containing groups onto the carbon surface. Boehm titration, potentiometric titration, thermal analysis, and sorption of nitrogen were used for detailed surface characterization. Valeric acid adsorption from aqueous solution was measured at 333 K. The calculated isotherms showed a good fitting to the Freundlich equation. The capacity coefficients revealed a direct correlation with the number of basic groups present on the surface. The amount of valeric acid adsorbed at its low concentration is dependent on the number of basic groups, whereas at high concentration the amount adsorbed depends on the volume of micropores smaller than 10 Angstrom, where the adsorption potential is the strongest. (C) 2003 Elsevier Inc. All rights reserved.