Abstract The solution chemistry of 1 : 2 : 4-triazole (TRZ) is investigated in the range of solution concentrations in which the compound acts as a corrosion inhibitor for copper. Parallel photo-electron spectroscopic studies show that although a surface layer of Cu(I) complex is formed at low TRZ concentrations, this is ineffective as an inhibitor until at a critical concentration a Cu(II) complex appears which provides effective protection. It is suggested that the precipitation of Cu(II) hydroxychloride is which the TRZ is attached provides the protective layer. Comparison is made between triazole and benztriazole as corrosion inhibitors for copper.