The potential of isotopic exchange of oxygen ¹⁸O in the studies of NOx decomposition over Fe-zeolites is illustrated and preceding papers of our group are summarized. Isotopic exchanges of ¹⁸O₂ for oxygens from decomposing nitrous oxide and of oxygen from N₂¹⁸O and N¹⁸O for zeolite oxygens are employed to study the heterogeneity of oxygen from nitrous oxide, the participation of zeolitic oxygens, and the manner of recombination of oxygen atoms. The oxygen transferring role of added NO during the decomposition of nitrous oxide is discussed. Attention is also paid to the effect of the zeolite matrix (ferrierite, BEA and ZSM-5) and to the effect of iron loading and location.