The spectroelectrochemical behavior of Cu in 1 M sodium acetate solution, pH 5.8, alone and in the presence of benzotriazole (BTA) was investigated by cyclic voltammetry, impedance spectroscopy, and Raman spectroscopy. Impedance spectra are characterized by two time constants relating to charge transfer and transport of copper ions through the surface film, the latter being the rate-determining factor. The finite diffusion impedance is analyzed using a diffusion factor (B), which is a function of the diffusion layer thickness (d) and the diffusion coefficient D-f for Cu+ species. The analysis suggests that changes in the film thickness can be monitored in situ by impedance spectroscopy. The values of D-f and concentration of Cu+ species (c(f)) in the film formed in the presence of 1 M BTA were estimated to be of the order 10(-14) cm(2) s(-1) and 10(-6) mol cm(-3), respectively. The results indicate that the film formed at higher inhibitor concentration and longer immersion time behaves almost like an ideal capacitor. Its thickness (up to 20 nm) and protection against corrosion increase by increasing the BTA concentration and immersion time. The results obtained by Raman spectroscopy confirm the model for the complex Cu(I)BTA in which overlap of Cu sp-hybrid atomic orbitals with the N-hybridized lone-pair atomic orbital occurs.