A steady-state isotopic transient analysis of NO adsorption over Co-ZSM-5 catalyst in the absence or in the presence of oxygen is reported by using doubly labeled nitric oxide. The kinetics and mechanism of NOX formation as well as the number of active sites for NO adsorption and their possible location on the catalyst surface are determined. The formation of NO2delta+ species, active intermediates in deNOX chemistry in the presence of oxygen and hydrocarbons, is shown to proceed according two sequential steps: (i) molecular oxygen adsorption and (ii) NO reaction with oxidized surface sites. The frequency factor of that reaction is found to be two orders of magnitude higher than that of mononitrosyls formation via the adsorption of NO on cobalt sites. The oxygen exchange within NO2delta+ is considerably slower than the formation of that intermediate species, indicating that its two oxygen atoms are nonequivalent. The concentration of active sites leading to NO2delta+ formation is also estimated and related to the interface existing between cobalt oxide clusters and zeolite framework.