The reduction of ClO4- ions in acid media was studied at smooth polycrystalline rhodium electrodes by voltammetry, chronoamperometry and impedance spectroscopy. On the basis of the experimental results. a new mechanism for the electrocatalytic reduction of ClO4- ions has been proposed. According to this reaction scheme. the reduction process is starting with the adsorption of perchlorate ions at free active sites on the metal surface. It is also assumed that the whole process starting from perchlorate ion occurs on the surface and the role of desorption of intermediates can be neglected. On the basis of the kinetic model an expression has been derived for the impedance of the electrode, which fits the experimental data quite well. (c) 2007 Elsevier Ltd. All rights reserved.