The electrochemical behaviour of Cu and Cu-10Ni alloy in 1 M sodium acetate solution, pH 5.8 without and in the presence of benzotriazole (BTAH) was investigated by cyclic voltammetry, photopotential measurements and impedance spectroscopy. Both polarization and impedance studies clearly show that BTAH forms the multilayer structure Cu/Cu2O-Cu(I)BTA on the electrode surface at a higher concentration of the inhibitor (c(BTAH) greater than or equal to 1 mM) and at longer immersion time. At low inhibitor concentration and a shorter immersion time molecules of the inhibitor are adsorbed chemically on the electrode surface. Several important properties df the adsorbed layer and the passivated films were determined from the impedance spectra. The system parameters obtained by a nonlinear fit procedure are discussed in terms of the model used. The results indicate that the surface film was of a dielectric nature. Its thickness and protection against corrosion increase with increasing inhibitor concentration and/or ageing. The adsorptive behaviour of BTAH on the electrode surface follows a Langmuir-type isotherm with the free energy of adsorption, Delta G = -36.7 kJ mol(-1). Photopotential measurements show that the surface layer formed on Cu in the presence of BTAH is stoichiometric, while the film on an alloy shows small non-stoichiometry, i.e. the excess of anion vacancies, n-type conductivity.