The effect of immobilized quercetin on activated carbon on Cu2+ and Ca2+ adsorption, as a function of a potential, was investigated. Ranges of potentials, corresponding to areas of selective adsorption of investigated cations, were obtained. Ca2+ ions do not adsorb on unmodified activated carbon AG-3V. In contrast, the modification of the activated carbon increases the adsorption activity of Ca2+. There are two areas of selective adsorption of Ca2+ ions. The selective adsorption of Cu2+ ions on unmodified sorbent was not found. The selective adsorption of Cu2+ ions on the quercetin-modified activated carbon initiates at potentials near E = -0.25 V. Dissolved oxygen and the acidification of a solution, containing calcium or copper ions, decrease the adsorption activity on modified and unmodified activated carbons. (c) 2005 The Electrochemical Society.