N2O decomposition on an ion-exchanged Fe-MFI catalyst has been studied using an O-18-tracer technique in order to reveal the reaction mechanism. (N2O)-O-16 was pulsed onto an O-18(2)-treated Fe-MFI catalyst at 693 K, and the O-2 molecules produced were monitored by means of mass spectrometry. The O-18 fraction in the produced oxygen had almost half the value of that on the surface oxygen, and (OO)-O-18-O-18 was not detected. The result shows that O-2 formation proceeds via the Eley-Rideal mechanism ((N2O)-O-16 + O-18(a) --> N-2 + (OO)-O-16-O-18).