The adsorption of different compounds such as gallic, tannic, and humic acids on an activated carbon at 298 K has been studied. The capacity of this carbon to adsorb gallic acid is much higher than for the other two acids, which has been explained on the basis of both their molecular size and ionization degree. The influence of dissolved Cr(NO3)(3) On the adsorption of these acids has been investigated. The capacity of the carbon to adsorb gallic and tannic acids in the presence of Cr(III) is slightly higher than in the absence of this metal. The variation of the surface properties of the carbon, both porous texture and charge, with the adsorption of these humic substances also has been tested. The porous texture changes have been studied from the N-2 adsorption isotherms and the changes in the carbon surface charge from the pH drift tests. The results indicate that the adsorption of these acids on the activated carbon brings about, to a greater or lesser extent, both pore blockage and an increase in the negative surface charge of the carbon. Finally, changes in the amount of Cr(III) adsorbed on the carbon as a function of the concentration of each of these three acids have been studied. A large decrease in the Cr(III) uptake is observed when these acids are present at low concentrations due to the pore blockage effect of the acid adsorbed on the carbon surface. When the acid concentration increases, the Cr(III) uptake also increases due to interactions of the Cr(III) cations with the negatively charged unbound functional groups of the adsorbed acid.