The inhibition mechanisms of benzotriazole and its 5-methyl, 5-chloro and 5-nitro derivatives were investigated for copper corrosion in sulphate solutions at several values of pH by an impedance technique and surface enhanced Raman scattering spectroscopy. Protective films of cuprous complex polymers of the inhibitors were readily formed on the copper surface at high pH values and noble potential. The ease of complex formation was estimated to decrease in the order, 5-chlorobenzotriazole > benzotriazole congruent-to 5-methylbenzotriazole > 5-nitrobenzotriazole. The order agreed with that of the inhibition efficiencies in the neutral solution.