The adsorption of Pb2+, Cd2+, Ni2+, Co2+ on activated carbon particles was studied as a function of pH and concentrations of adsorbates. These results showed that the adsorption was strongly pH-dependent and all the adsorption isotherms were in agreement with Freundlich equation. The adsorption could be explained by the surface complexation model. Conditional adsorption constants of surface complexes of Pb2+, Cd2+, Ni2+ and Co2+ with activated carbon particles were calculated by a simplified surface complexation model.