Steady-state isotopic transient kinetic analysis (SSITKA) coupled with temperature-programmed surface reaction (TPSR) methods and using in situ mass spectroscopy and DRIFTS have been applied for the first time to study essential mechanistic aspects of the NO/H₂/O₂ reaction at 140 °C under strongly oxidizing conditions over 0.1 wt % Pt/SiO₂ and 0.1 wt % Pt/La−Ce−Mn−O catalysts. The nitrogen-pathway of the reaction from NO to form N₂ and N₂O gas products was probed by following the ¹⁴NO/H₂/O₂ → ¹⁵NO/H₂/O₂ isotopic switch at 1 bar total pressure. It was found that the chemical structure of active intermediate NO_x species strongly depends on support chemical composition. In the case of the Pt/SiO₂ catalyst, the reaction route for N₂ and N₂O formation passes through the interaction of one reversibly and one irreversibly NO_x species chemisorbed on the Pt surface. On the other hand, in the case of a Pt/La−Ce−Mn−O catalyst, the reaction route passes through the interaction of two different in structure irreversibly chemisorbed NO_x species on the support. For the latter catalyst, the mechanism of the reaction must involve a hydrogen-spillover process from the Pt metal to the support surface. A surface coverage θ = 1.8 (based on Pt metal surface) of active NO_x intermediate species was found for the Pt/La−Ce−Mn−O catalyst. A large fraction of it (81.5%) participates in the reaction path for N₂ formation, whereas in the case of Pt/SiO₂, this fraction was found to be 68.4% (active NO_x, θ = 0.65). These important results provide an explanation for the lower N₂ reaction selectivity values observed on Pt/SiO₂ compared to Pt/La−Ce−Mn−O catalyst. Inactive adsorbed NO_x species (spectators) were found to accumulate on both Pt and support surfaces. It was found via the NO/H₂/¹⁶O₂ → NO/H₂/¹⁸O₂ isotopic switch that the reaction path from NO to form N₂O passes through the oxidation step of NO to NO₂ with the participation of gaseous O₂, where the extent of it depends on support chemical composition.