We,have carried out a CASSCF and CASPT2 ab initio study of the N(D-2) + O-2 system, that is very relevant in the upper atmosphere, The saddle points of the PESs involved in the two reactions that can occur (O(D-1) + NO (1) and O(P-3) + NO (2)) have been characterized. According to TST and VTST calculations performed, the N(D-2) + O-2 reactivity takes place through reaction (2). Also, a good agreement with the experimental kinetics data corresponding to the global deactivation of N(D-2) has been obtained. These results suggest that reaction (2) dominates over the physical electronic quenching of N(D-2).