The influence of benzotriazole (BTAH) and tolyltriazole (TTAH) on the cathodic reduction of copper(II) at poly-oriented platinum from acidic chloride solutions has been studied by rotating disk voltammetry and cyclic voltammetry combined with electrochemical quartz crystal microbalance (EQCM). The overall electrode process involves two one-electron reactions: Cu(II)/Cu(I) and Cu(I)/Cu, which art: characterized by a very large potential separation. In the additive-free solutions, the first electrode reaction is slightly inhibited by underpotential deposited Cu. On the basis of EQCM results, it has been proved that the interaction of BTAH and TTAH with the electrode surface is potential dependent. The adlayers of the inhibitors are removed in the potential range below -0.25 vs. SCE, in which the adsorption of hydrogen occurs. Consequently, the Cu(II)/Cu(I) electrode reaction is strongly inhibited by BTAH and TTAH while the Cu(I)/Cu reaction remains practically unaffected.