The adsorption of inorganic anions, namely, bromate, chloride, chlorate, iodate, nitrate, perchlorate, dihydrogen phosphate, and sulfate on activated carbon was studied. The free energy of adsorption was calculated by including chemical interaction, solvation, and electrostatic attraction. Results showed that specific chemical interaction between anions and the activated carbon surface played a major role on the adsorption process. Attempt was made to correlate solute properties such as hydration number, hydration enthalpy, and free energy of hydration to the degree of adsorption. Results showed good linear relationship between the free energy of adsorption and the properties of the solutes, specifically the hydration number, the free energy of hydration and the hydration enthalpy for all anions studied with a correlation coefficient (R-2) of 0.97, 0.87, and 0.86, respectively, except sulfate and dihydrogen phosphate. The results suggested that the adsorption of sulfate and dihydrogenphosphate followed mechanisms different from that of other oxyanions, which mostly are the conjugate bases of strong BrOnsted acids. The competition between perchlorate and other anions for the activated carbon was studied as well. Among all inorganic anions studied and in the concentration range of 0.1-1.0 mM, only nitrate competed with perchlorate for adsorption. A competition model based on the Langmuirian type was adequate to describe the adsorption of oxyanions in bisolute system on activated carbon. (C) 2011 Elsevier B.V. All rights reserved.