N2O decomposition on over-exchanged Cu-ZSM-5 zeolite was investigated in the range of temperatures from 323 to 400 degrees C. When only N2O in helium was fed, regular periodic oscillations of the outlet concentration of N2O were observed. Catalyst pretreatment affected the pattern of the oscillations, but not their periods or amplitudes. Also the average conversion of N2O to N-2 and O-2 was dependent on the initial oxidation state of the catalytic active sites, being lower when the catalyst was pretreated in oxygen. The addition of NO, O-2 or both in the feed resulted in disappearance of the oscillations and decrease of N2O conversion. The experimental observations are in agreement with a scheme of reactions in which N2O acts either as oxidizing or as reducing agent for the catalytic sites.