Abstract Surface films formed by adsorption of benzotriazole (BTA), on copper and copper alloys have been studied by X-ray photo-electron spectroscopy (XPS). It is found that on both copper and copper-nickel alloys, benzotriazole forms a Cu(I)BTA surface complex. For the copper-nickel alloys the time taken to form the Cu(I)BTA film is dependent on the alloy composition. The Cu(I)BTA films were found to oxidize rapidly to a Cu(II) species on removal from the liquid phase. Surface films formed by BTA on 70 Cu : 30 Zn alloy were found to contain both copper and zinc, the copper again being in the Cu(I) state. A study of the adsorption of BTA on cathodically reduced copper surfaces strongly supports previous suggestions that the presence of Cu 3O facilitates formation of the surface film.