The thermodynamics of the so-called perfectly polarized electrode were employed to analyze the total charge densities for a nearly defect-free Pt(1 1 1) electrode in NaCl solutions with an excess of an inert electrolyte (0.1 M HClO4). A complete thermodynamic analysis using the electrode potential and the charge as independent variables has been performed. The Gibbs excess, Gibbs energy of adsorption, charge numbers at constant electrode potential and constant chemical potential for chloride adsorption at the Pt(1 1 1) surface have been determined. Our results show that the polarity of the chemisorption bond is very small. The highest packing density of chloride determined by the thermodynamic method corresponds to 0.5 ML and is only 10% smaller than the coverage expected for a close packed monolayer of chlorine atoms. We demonstrated that at negative potentials, where Cl and hydrogen adsorption overlaps, the adsorption has a competitive character. (C) 2004 Elsevier B.V. All rights reserved.