The effect of benzotriazole (BTA) on corrosion of copper in 5% NaCl has been studied by immersion tests, potentiostatic polarization, AC impedance measurements, and ellipsometric investigations. It was found that BTA when present in the corrosive environment is a good inhibitor. The good electronic conductivity of the film formed on copper, the decreased double capacity, small film thickness, and decreased current on anodic polarization curves indicate that BTA is chemisorbed on the surface. This prevents adsorption of oxygen and formation of a prenucleation layer which is the forerunner of oxide formation. Discrepancies with results obtained by other investigators using infrared reflectance techniques are discussed.