The kinetics of interaction of benzotriazole (C6H5N3, BTAH) with the surface of copper in salt water were studied using an electrochemical quartz crystal microbalance and X-ray photoelectron spectroscopy (XPS). Upon injecting BTAH into the electrolyte, three regions appear in the time response of the microbalance. Region I (at short time of few minutes), exhibits rapid linear growth of mass with time, which is attributed to the formation of a protective Cu(I)BTA complex. Region II reveals attachment of BTAH at a slower rate onto the inner Cu(I)BTA complex. Region III is a plateau indicating that the BTAH film attains an equilibrium mass and thickness, which increase with the concentration of BTAH. The intensity of the N1s peak in the XPS spectra increases with the time of immersion, indicating more BTAH on the surface. The results suggest a duplex inhibitor film composed of an inner thin layer of Cu(I)BTA and an outer layer of physically adsorbed BTAH which increases in thickness with time and BTAH concentration. They also offer an explanation for the much documented findings of simultaneous increase of the polarization resistance and decrease of double layer capacity with inhibitor concentration and time of immersion.