The dynamics of N2O decompn. to gaseous nitrogen and oxygen over HZSM-5 catalysts with a low iron content (200 and 1000 ppm) was studied by the transient response. Method in the temp. range 523-653 K. The active catalysts were prepd. from HZSM-5 with Fe in the framework on its steaming at 823 K followed by thermal activation in He at 1323 K. Two main steps were distinguished in the dynamics of N2O decompn. The first step represents N2O decompn. forming gaseous nitrogen and surface at. oxygen. The second step is assocd. with surface oxygen recombination and desorption. At 523-553 K only the first step is obsd. Above 573 K the decompn. of N2O to O2 and N2 in stoichiometric amts. starts at a rate increasing with time until a steady-state value is reached. This increase was assigned to the catalysis by adsorbed NO formed slowly on the catalyst surface from N2O, as indicated by temp.-programmed desorption. The catalytic effect of the adsorbed NO was also confirmed by transient expts. with forced addn. of NO in the stream of N2O during its decompn. A simplified kinetic model is proposed to explain the autocatalytic reaction. Catalyst pretreatment in O2 did not affect N2O decompn., but irreversible water vapor adsorption at 603 K resulted in a twofold decrease in surface oxygen loading from N2O and complete inhibition of the oxygen desorption. [on SciFinder (R)]