The selective catalytic reduction (SCR) of NO (2000 ppm) by NH3 (2000 ppm) in the presence of oxygen (3%) was carried out on Cu(x)-FAU (x = theoretical exchange degree) catalysts prepared by ion exchange or impregnation and calcined at 773 K. The samples were characterized by UV-visible and IR spectroscopy, temperature-programmed reduction (TPR), temperature-programmed oxidation (TPO), and temperature-programmed desorption (TPD) of NH3. Ion-exchanged Cu(x)-FAU contains mainly Cu ions located in both supercages and sodalite cavities. In contrast, the impregnated sample contains mainly CuO. Ionic Cu is more active and selective to N-2 than CuO in the temperature range 450-750 K. In contrast, CuO aggregates lead to significant formation of N2O, with a bell-shaped dependency centered at ca. 540 K. IR spectroscopy and TPD of NH3 show that the last NH3 ligand was removed from Cu ions above 550 K. The SCR on Cu ions obeys a Cu2+ <-> Cu+ redox mechanism in which Cu2+ is reduced to Cu+ by NO + NH3 and Cu+ is oxidized to Cu2+ by NO + O-2, with evolution of N-2 and H2O. Both reduction and oxidation steps of Cu in the catalytic cycle encompass the reduction of NO in agreement with the SCR of (NO)-N-14 with (NH3)-N-15. A new overall SCR reaction below 550 K was proposed: 10NH(3) + 13NO + O-2 --> 15H(2)O --> (23/2)N-2. The active sites below 550 K are formed by several Cu neighbor ions, maybe [CuOCu](2+), probably located in the supercages. All Cu ions become active above 600 K. The partial reduction of NO to N2O occurs at high temperature (>650 K) on exchanged samples. This formation, up to 17% at full NO conversion, is likely to take place on Cu ions located within the sodalite cavities.