Kinetics and mechanism of nitrate and nitrite reduction on Pt(100) electrode modified by Cu adatoms have been studied in solutions of sulfuric and perchloric acids by means of cyclic voltammetry and in situ IR-spectroscopy. It has been shown that the surface redox process with participation of ammonia or hydroxylamine at 0.5–0.9 V occurs only on the Cu-free platinum. The causes of this effect could be low adsorption energy of nitrate reduction products on copper or changes in the composition of the products (ammonia for Pt(100) and N2O for Pt(100)+Cu). Nitrate reduction on Pt(100)+Cu electrode is much faster in the perchloric acid solution (by several orders of magnitude) as compared with unmodified platinum as a result of induced adsorption of nitrate anions in the presence of partly charged Cu atoms. In the solutions of sulfuric acid the rate of nitrate reduction is considerably lower as copper adatoms facilitate adsorption of sulfate anions, which block the adsorption sites for the nitrate.