Isothermal oscillations in the rate of decomposition of N2O were studied on an over-exchanged Cu-ZSM-5 catalyst by mass spectroscopy and in situ transient FTIR. Oscillations in the production of O-2 and N-2 were observed to occur in a temperature range of 410-490 degrees 'C at a total pressure of 1.0 Torr pure N2O. FTIR has provided them first spectroscopic evidenced that surface nitrate species are present under oscillatory conditions. This study confirmed a previously proposed model that predicts a slow build-up of surface nitrates, followed by a rapid nitrate decomposition coupled with an increase in the rate of N2O decomposition. The IR signature of the surface nitrates suggests they are monodentate nitrate species bound to Cu2+ ions. Temperature-programmed desorption studies reveal a strong correlation between the stability of the surface nitrate species and the temperature range in which oscillations occur.