Combined EQCM and voltammetric studies in conjunction with radiotracer adsorption investigations were carried out in order to obtain direct information on the interfacial behavior of the Cu2+ -Cu system under equilibrium and dynamic conditions with the aim of clarifying the role and participation of Cu+ ions formed by the interaction of cupric ions and copper metal. The appearance and accumulation of Cu+ ions in the solution phase is clearly demonstrated through radiotracer experiments using labeled Cl- ions. The formation of bulk CuCl phase is preceded by the formation of a CuCl surface layer. Data obtained from combined voltammetric and EQCM studies allow us to separate the potential regimes where the electrosorption, the formation of Cu+ or Cu2+ ions, and dissolution of CuCl are the predominant processes.