Pulsed N2O decomposition on a reduced or oxidized Rh/USY catalyst has been carried out to study the dependence of oxygen coverage (theta (o)) on activity and to study the mechanism of O-2 desorption using an isotopic tracer technique. Decomposition activity decreased to the minimum (formation of only N-2) at theta (o) = 0.5, but increased with increasing coverage (theta (o) > 0.5), and finally high activity with steady-state O-2 production was attained at high theta (o) (>1.2). In the isotope study, N-2 O-16 was pulsed onto O-18/oxidized Rh catalyst at low temperatures (220-240 degreesC), and desorbed O-2 molecules were monitored by means of mass spectrometry. The O-18 fraction in the desorbed oxygen had almost the same value as that on the surface oxygen. The result shows that O-2 desorption does not proceed via the Eley-Rideal mechanism, but via the Langmuir-Hinshelwood mechanism, i.e., desorption of dioxygen through the recombination of adsorbed oxygen. On the other hand, O-2-TPD measurements in He showed that desorption of oxygen from the Rh catalyst occurred at much higher temperatures. Therefore, it was proposed that reaction-assisted desorption of O-2 occurs during N2O decomposition at low temperatures,