Steady-state isotopic transient kinetic analysis (SSITKA) was applied to study the reactivity of adsorbed NOx species with methane over Co-ZSM-5 during CH4-SCR of NO at different partial oxygen concentrations. Numerical analysis of the isotope responses after switching from (NO)-N-14-O-16 to (NO)-N-15-O-18, from O-16(2) to O-18(2), from (CH4)-C-12 to (CH4)-C-13, and from (CO2)-O-16 to (CO2)-O-18 in the feed gas was performed. This enables determination of (i) the concentration of different forms of NOx adspecies on Co sites, (ii) their reaction rates with methane, and (iii) the concentration of surface intermediates resulting from the interaction of NOx species with methane. The NO reduction with CH4 in the presence of oxygen was shown to proceed by two different pathways with participation of (1) NO2delta+ species (formed on nanoclusters of cobalt oxide located inside the zeolite channels) and (2) NO2- nitrite complexes (formed on larger cobalt oxide particles located outside the zeolite channels), the reaction rate by the former being appreciably higher under the SCR reaction conditions studied. Mononitrosyl species appear not to be directly involved in the overall process. Modeling results indicate that the rate of the first route is limited by the interaction of NO2delta+ species and adsorbed methane, while in the second route the formation of nitrite species is a rate-determining step. Based on the obtained results, an overall reaction mechanism of the CH4-SCR of NO in the presence of oxygen is proposed. (C) 2004 Elsevier Inc. All rights reserved.