Isotope exchange (I. E) of O-18(2) for oxygen captured in Fe-FER after decomposition of nitrous oxide proceeds readily even at room temperature, provided that the amount of surface NOx species, formed as a secondary product of the N2O decomposition, is relatively low. If O-18(2) is present during the decomposition of nitrous oxide above 250 degrees C, or if nitrous oxide labeled with O-18 is employed, O-18 appears also in NOx species, and I. E easily occurs with zeolite framework oxygens.