Cyclic voltammetry, differential capacity determinations and chronocoulometry have been employed for quantitative study of thiourea (TU) adsorption at polycrystalline gold/aqueous KClO4 solution interface. The film pressure and the Gibbs surface excess are determined as functions of potential. The thermodynamic data suggest that the adsorbed TU exhibits distinct surface states which appear when the TU concentration is increased and the potential is more positive. (C) 2003 Elsevier Science Ltd. All rights reserved.