N2O decomposition on an oxidized Rh catalyst (unsupported) has been studied using a tracer technique in order to reveal the reaction mechanism. (N2O)-O-16 was pulsed onto an O-18/oxidized Rh catalyst at 493 K and desorbed O-2 molecules were monitored. The O-18 fraction in the desorbed oxygen had the same value as that on the surface oxygen. The result shows that the oxygen molecules do not desorb via the Eley-Rideal mechanism, but via the Langmuir-Hinshelwood mechanism. On the other hand, desorption of oxygen from Rh surfaces (in vacuum or in He) occurs at higher temperatures, which suggests reaction-assisted desorption of oxygen during the N2O decomposition reaction at low temperature.