A thermodynamic method based on the work done by Frumkin and Petrii [A.N. Frumkin, O.A. Petrii, Electrochim. Acta 20 (1975) 347], to calculate the so-called double layer capacity for a Pt(111) electrode is proposed. The analysis requires careful measurement of the total charge density versus potential curves for a series of solutions with composition (0.1 - x) M KClO4 + xM HClO4. A method in which the total charge densities are determined by integration of cyclic voltammograms recorded in solutions with or without chloride is described. Following this procedure the double layer capacity curves were calculated. The double layer capacity curves displayed three peaks that were tentatively assigned to the solvent reorientation, onset of OH adsorption and completion of the OH adlayer. In the hydrogen adsorption region, the double layer capacity values were 14 +/- 5 mu F/cm(2), in good agreement with previous estimates reported in the literature by using other approaches. (c) 2005 Elsevier Ltd. All rights reserved.