The influence of benzotriazole, at concentrations to 10 g l(-1), on the efficiency of anodizing of Al-3.5 wt% Cu alloy at constant current density in 0.1 M ammonium pentaborate electrolyte has been investigated for two conditions of the alloy, namely solution-treated and solution treated and artificially aged. For the solution-treated alloy, the addition of benzotriazole has negligible effect on the voltage-time response and, hence, also the efficiency of film growth, which was about 92% for film formed to 200 V. In contrast, benzotriazole increases significantly the efficiency of film growth on the solution-treated and aged alloy, from similar to 45 to similar to 70% for films formed to 200 V. The increased efficiency is attributed mainly to the inhibition of oxygen evolution on CuAl2 particles by formation of a polymeric, Cu-I-benzotriazole film on the copper-rich phase. The influence of benzotriazole, from comparisons of voltage-time responses for both heat treatments of the alloy, is greatest at the commencement of film growth and subsequently diminishes, with little, or no, effect following film formation to 150 V. The diminishing influence is associated with eventual growth of relatively thick anodic film, which limits oxygen evolution.