In the context of decreasing the emissions of greenhouse gases, a Fe-exchanged zeolite-beta (Fe-BEA) catalyst is shown to be very active in the reduction of N2O by NH3 in the presence of O-2. The temperature at which 50% N2O conversion is obtained is lower by ca. 80 K compared to its catalytic decomposition in the absence of NH3. TPR, TPO, and TPD experiments after treatments in various atmospheres provide evidence that the reaction involves the redox cycle Fe-III <->Fe-II where the Fem active species are Fe oxocations of low nuclearity. N2O decomposes into O* surface species on specific reduced Fe sites with the concurrent release of N-2; these species do not compete with O* coming from O-2 for their removal by NH3. In the absence of O-2, catalytic experiments with (N2O)-N-14 and (NH3)-N-15 show that: (1) N-2 is mainly formed from (NN)-N-14-N-14-O Splitting to yield N-14(2), the O* species being in turn removed by (NH3)-N-15 to give N-15(2); (2) some N-14-(NO)-N-14 bond splitting occurs, which leads to (NN)-N-14-N-15 after reaction of (NO)-N-14* and (NH3)-N-15 through a classical SCR mechanism. The Fe active species in the N-NO splitting are inhibited in the presence of O-2. The kinetics of N2O reduction by NH3 obeys a Mars and van Krevelen oxide-reduction mechanism modified with an inhibiting term of NH3.