The kinetics of the selective catalytic reduction (SCR) of NO by NH 3 in the presence of O 2 has been studied on a 5.5% Cu-faujasite (Cu-FAU) catalyst. Cu-FAU was composed of cationic and oxocationic Cu species. The SCR was studied in a gas phase-flowing reactor operating at atmospheric pressure. The reaction conditions explored were: 458<T R <513K, 250<NO (ppm) < 3000, 1000<NH 3 (ppm) < 4000, 1<O 2 (%) < 4. The kinetic orders were 0.8-1 with respect to NO, 0.5-1 with respect to O 2 , and essentially 0 with respect to NH 3 . Based on these kinetic partial orders of reactions and elementary chemistry, a wide variety of mechanisms were explored, and different rate laws were derived. The best fit between the measured and calculated rates for the SCR of NO by NH 3 was obtained with a rate law derived from a redox Mars and van Krevelen mechanism. The catalytic cycle is described by a sequence of three reactions: (i) Cu I is oxidized by O 2 to ''Cu I I -oxo'', (ii) ''Cu I I -oxo'' reacts with NO to yield ''Cu I I -N x O y '', and (iii) finally ''Cu I I -N x O y '' is reduced by NH 3 to give N 2 , H 2 O, and the regeneration of Cu I (closing of the catalytic cycle). The rate constants of the three steps have been determined at 458, 483, and 513K. It is shown that Cu I or ''Cu I I -oxo'' species constitute the rate-determining active center.