The effect of an NO isotope on the kinetics of lean-NOx catalysis in (NO + C2H4 + O-2) reaction mixtures over Pt-ZSM-5 was theoretically analyzed and compared with experimental data obtained with a fixed-bed plug-flow reactor. The large kinetic isotope effect, as well as kinetic oscillations observed previously, was explained by NO decomposition kinetics on Pt surfaces which undergo phase transition between (1 x 1) and (hex) phases induced by adsorbed reactant species. Results indicate the importance of NO decomposition kinetics in lean-NOx catalysis, lending further support to a lean-NOx reduction mechanism which involves NO decomposition accompanied by hydrocarbon oxidation.