The adsorption of cadmium and zinc ions, IIb family heavy metals, from aqueous solutions onto the two activated carbons of different surface nature, one with acidic functional groups prepared by 95°C concentrated nitric acid oxidation (oxidized carbon) and the other with no acidic functional group treated at 1000°C in helium flow (out-gassed carbon), was investigated to clarify the influence of background electrolyte on the adsorption performance. The cadmium uptake onto oxidized carbon was found to be decreased under the coexistence of NaCl, because Na⁺ occupied adsorption sites for cadmium. For the out-gassed carbon, the cadmium uptake was increased at low pH due to the formation of complex anions of CdCl₃⁻ and CdCl₄²⁻, but zinc uptake was not changed any PH, because chloride anions could not make complex with zinc ions. Under the coexistence of NaNO₃, the increase in cadmium uptake to the out-gassed carbon was not observed because the complex is no longer formed with nitrate anions for cadmium. At simultaneous adsorption of cadmium and zinc ions to the outgassed carbon under the coexistence of NaCl, only cadmium could be adsorbed at low pH as expected. The results suggest that the separation of these IIb family ions may be achieved using out-gassed carbon.