We present direct evidence for the formation of nitrites/HONO in the oxidative activation of NO over Fe-zeolites. Exposure of a Fe-ZSM-5+BaO/Al2O3 physical mixture to NO+O2 at 120°C resulted in forming nitrites on Fe-sites and trapping them on BaO via gas-phase equilibrium with HONO. Identification of nitrites was confirmed by: (i) their thermal decomposition to an equimolar mix of NO and NO2 during TPD, (ii) N2 formation during their reaction with NH3 at 120°C, (iii) ex situ IR analysis of the BaO powder after discharge and separation of the mechanical mixture. The reactivity data suggest that nitrites/HONO are intermediates both in the NO oxidation to NO2 and in the Standard SCR reactions.