The selective catalytic reduction (SCR) of NOx (NO+NO2) by NH3 in O-2 rich atmosphere has been studied on Cu-FAU catalysts with Cu nominal exchange degree from 25 to 195%. NO2 promotes the NO conversion at NO/NO2=1 and low Cu content. This is in agreement with next-nearest-neighbor (NNN) Cu ions as the most active sites and with NxOy adsorbed species formed between NO and NO2 as a key intermediate. Special attention was paid to the origin of N2O formation. CuO aggregates form 40-50% of N2O at ca. 550 K and become inactive for the SCR above 650 K. NNN Cu ions located within the sodalite cages are active for N2O formation above 600 K. This formation is greatly enhanced when NO2 is present in the feed, and originated from the interaction between NO (or NO2) and NH3. The introduction of selected co-cations, e.g. Ba, reduces very significantly this N2O formation. (C)1999 Elsevier Science B.V. All rights reserved.