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In this study, we compared the adsorption of methionine, cysteine and cystine at the mercury/chlorate (VII) interface. Changes in the surface tension with the potential of the zero charge γ z and relative surface excess Γ' of methionine, cysteine and cystine as a function of their concentration indicate that the adsorption of the studied amino acids increases in the order of methionine > cystine > cysteine. For all the systems examined, linear dependences ln[×(1–Θ)/Θ = f(Θ)] (Frumkin isotherm) and ln (Γ/c) = f(Γ') (virial isotherm) were obtained. Changes in the adsorption parameters as a function of the supporting electrolyte concentration indicate the competitive adsorption of amino acids and ClO 4 − ions and electrostatic interactions between the water dipoles and adsorbates in the double layer.