Chronocoulometry was employed to measure charge density at the Au(111) electrode surface. Thermodynamic analysis of the charge density data for ionic adsorption from a solution with an excess of a supporting electrolyte is described and used to study SO42- adsorption at the Au(111) electrode from a 0.1 M HClO4 solution. The Gibbs excesses determined from the charge density data are compared with the Gibbs excesses determined from radiochemical measurements using S-35-labelled sulphate solutions. Very good agreement between the Gibbs excesses determined by the two techniques is observed. Essin-Markov coefficients for SO42- adsorption from two series of solutions were determined : (i) at constant pH and variable K2SO4 concentration; (ii) at constant K2SO4 concentration and variable pH. The Essin-Markov coefficients were used to identify the nature of the adsorbed species (HSO4- or SO42-). Our results indicate that SO42- ion is the adsorbed species even if HSO4- predominates in the bulk of the solution.