Decomposition of NO was studied on Cu-ZSM-5 catalysts prepared by solid state ion exchange using CuCl2 (I), CuO (II) and by conventional liquid phase ion exchange with copper acetate (III). There was no difference in the catalytic activity among samples (I), (II) and (III) using the same copper loading. Treatment of the samples in argon, in air or in NO/Ar mixture at 700 degrees C was necessary to develop optimum catalytic activity. Transient kinetic experiments using NO carried out under isothermal conditions, showed overshoots in the N-2 and O-2 concentration at the front and tail edge, respectively. Fourier transform-infrared studies indicated the formation of oxidized copper sites and adsorbed NO2 species during the NO decomposition. In a proposed mechanism Cu2+(O)(NO)(NO2) intermediate was suggested to play a key role in the NO decomposition.