[en] Agreement between isotherm equations and the law of active masses was obtained by studying the adsorption isotherms of cerium and terbium on an electrochemically oxidized graphite (EOG)in a.1 and 0.2 N HClO₄ and H₂SO₄ solutions. This confirms the ion-exchange mechanism of the cation adsorption on EOG. The equations of adsorption isotherms, the pattern of rate of isotopic half-exchange and equilibrium adsorption versus pH showed that upon adsorption of REE (at concentrations below 4.10^-4 g-ion/1) in 0.1-0.3 N HClO₄ solutions one cation replaces 2 H⁺ ions from EOG, while 0.1-0.2 N H₂SO₄ 3 protons are replaced. In the former case the MeOH²⁺ cations are generated and adsorbed at EOG while in the second case predominantly Me³⁺ ions are adsorbed. Different structure of a double EOG layer enables to explain these results. It was shown that differently hydrolyzed REE cations may be adsorbed at EOG with comparable rates up to pH of complete precipitation of hydroxides Me(OH)₃