The kinetics of N2O decomposition to gaseous nitrogen and oxygen over HZSM-5 catalysts with low content of iron (<400 ppm) under transient and steady-state conditions was investigated in the temperature range of 250–380 °C. The catalysts were prepared from the HZSM-5 with Fe in the framework upon steaming at 550 °C followed by thermal activation in He at 1050 °C. The N2O decomposition began at 280 °C. The reaction kinetics was first order towards N2O during the transient period, and of zero order under steady-state conditions. The increase of the reaction rate with time (autocatalytic behaviour) was observed up to the steady state. This increase was assigned to the catalysis by adsorbed NO formed slowly on the zeolite surface from N2O. The formation of NO was confirmed by temperature-programmed desorption at temperatures >360 °C. The amount of surface NO during the transient increases with the reaction temperature, the reaction time, and the N2O concentration in the gas phase up to a maximum value. The maximum amount of surface NO was found to be independent on the temperature and N2O concentration in the gas phase. This leads to a first-order N2O decomposition during the transient period, and to a zero-order under steady state. A kinetic model is proposed for the autocatalytic reaction. The simulated concentration–time profiles were consistent with the experimental data under transient as well as under steady-state conditions giving a proof for the kinetic model suggested in this study.