ONIOM calculations for the direct catalytic decomposition mechanism of NO2 and NO by Cu-ZSM-5 were carried out. In two layer calculations, Density Functional Theory and Universal Force Field were employed for the high and low level models, respectively. Delta H degrees and Delta G degrees evaluations were performed in order to determine the thermodynamically more favored way for catalytic decomposition of NO2 and NO. The results show that a novel copper kappa(2) mononitrosyl species (Z-Cu-2-kappa(NO)-N-2) is in equilibrium with the Z-(CuNO)-Cu-2 and Z-(CuON)-Cu-2 species. According to our results, the Z-Cu-2-kappa(NO)-N-2 species is the intermediary key of the direct catalytic decomposition mechanism of NO2 and NO by Cu-ZSM-5. (C) 2011 Elsevier B.V. All rights reserved.