The reaction mechanism of the fast selective catalytic reduction (SCR) of NO(x) has been investigated by means of a microkinetic model based on DFT calculations. First, potential additional reaction mechanisms are presented that include the intermediate formation of Z(-)[NO(x)](+) (x = 1, 2) from the decomposition of N(2)O(y) (y = 3, 4) on a Bronsted acid. The formed NO(x)(+) reacts with ammonia to nirosamine or nitramide. The DFT results are applied in a microkinetic model together with prior studies of Bruggemann et al. The simulated conversion of NO, is in agreement with experimental data over a wide range of temperature. The activity of the H-ZSM5 for the fast SCR is based on the reaction sequence via Z(-)[NO(x)](+), the decomposition of nitrosamine and nitramide, and the reaction of nitrous and nitric acid with adsorbed ammonia at low temperatures. Ammonia blocks the active sites at low temperatures while thermodynamic limitations of the surface species N(2)O(y), restrict the conversion at high temperatures. Heat of formations and reaction rate constants were adjusted within the accuracy of the applied method for important elementary steps to cope with experimental data. (C) 2011 Elsevier Inc. All rights reserved.