Thermodynamic analysis of charge density data was performed to describe sulfate adsorption at the Au(111) surface. The Gibbs excess, the Gibbs energy of adsorption, the number of electrons flowing to the interface per one adsorbed sulfate ion and the Esin-Markov coefficients were determined. The thermodynamic data indicate that sulfate adsorption at the Au(111) electrode has an ionic character and is driven by the interaction of the ion with its image charge on the metal. The surface dipole formed by the ion and its image is heavily screened by the metal and surface solvent molecules. The ionic character of sulfate adsorption at the Au(111) electrode is consistent with the results of independent second harmonic generation experiments. The second harmonic generation data indicate that adsorption of sulfate affects little the electronic structure of the metal surface.