The thermodynamics of the so-called perfectly polarized electrode were employed to analyze the total charge densities for a Pt(1 1 1) electrode in a series of solutions (0.1 - x) M HClO4 + x M KClO4 with a constant ionic strength and a variable hydronium ion concentration. The total charge densities were calculated by integration of cyclic voltammetry curves. A complete thermodynamic analysis using charge and potential as independent variables has been performed. The Gibbs excesses of adsorbed hydrogen and OH were determined without the need to introduce an arbitrary correction for the so-called charging of the double layer and further assumptions about the charge number per adsorbed species. We found that the maximum surface concentration of H amounts to similar to11 x 10(14) ions cm(-2) or to a coverage 0.73 monolayer (ML) in good agreement with previous estimates. The maximum surface coverage of OH is equal to similar to 8 x 10(14) ions cm(-2) or to a coverage similar to 0.53 ML. Using the thermodynamic method, the charge numbers at a constant potential (electrosorption valency) and at a constant chemical potential (reciprocal of the Esin-Markov coefficient) for adsorbed H and OH were calculated using the Gibbs excess data. The charge numbers are equal to similar to 1 for H and to similar to - 1 for OH. These values suggest that the adsorbed species is hydrogen atom and discharged OH species. (C) 2003 Elsevier B.V. All rights reserved.