The geometries and harmonic vibrational frequencies of several possible intermediates involved in the NO decomposition mechanism catalyzed by Cu-ZSM5: ZCu(NO2)(NO), ZCU(N2O3), ZCu(N2O)(O-2), Ku(N2O), and ZCU(O-2), have been determined using the B3LYP density functional approach and a tritetrahedral model for the zeolite. Results show that, although ZCu(NO2)(NO) and ZCu(N2O3) species are very stable with respect to the ground state ZCuO + 2NO asymptote, the NO decomposition is not expected to proceed through these intermediates. At high temperatures, formation Of ZCU(02) may occur from ZCuO + 2NO through a ZCu(OONNO) like transition state in the triplet surface. This pathway may compete with the one previously reported for the O-transfer to ZCuO frorn N2O and, thus, should be considered to understand the observed reactivity.