Isothermal oscillations during the catalytic decomposition of nitrous oxide over copper-exchanged ZSM-5 have been studied at 698 K employing mass spectrometry. Catalysts with a copper exchange level greater than 100% are more active for N2O decomposition and only these samples exhibit sustained oscillations. Two states of the catalysts with different activity toward the formation of N-2 could be identified. Transient experiments by using step changes in nitrous oxide concentration showed that large amounts of adsorbed species are present on the catalyst surface under working conditions. It is proposed that atomic oxygen and nitrate are formed by reaction of nitrous oxide. The amounts of adspecies change during oscillations and mirror the development of gas-phase concentrations. While gas-phase oxygen had no effect on the observed phenomena, addition of NO and other reducing agents such as H-2 and CO resulted in disappearance of the oscillations and maintained the catalyst in the more active state. The results suggest that the active species for nitrous oxide decomposition is monovalent copper and that the periodic oxidation and reduction of active sites is responsible for the development of kinetic oscillations.