N2O decomposition on an unsupported Rh catalyst has been studied using tracer technique in order to reveal the reaction mechanism. (N2O)-O-16 was pulsed onto O-18/oxidized Rh catalyst at 220 degreesC and desorbed O-2 molecules (m/e = 32, 34, 36) were monitored by means of mass spectrometer. The O-18 fraction in the desorbed dioxygen was the same value as that on the surface oxygen. The result shows that the O-2 molecules desorb via Langmuir-Hinshelwood mechanism, i.e., the desorption of dioxygen through the recombination of adsorbed oxygen. On the other hand, TPD measurements in He showed that desorption of oxygen from the Rh black catalyst occurred at the higher temperatures. Therefore, reaction-assisted desorption of oxygen during N2O decomposition reaction at the low temperature was proposed. (C) 2000 Elsevier Science B.V. All rights reserved.