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 + O-2 at 120 degrees 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) N-2 formation during their reaction with NH3 at 120 degrees 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. (C) 2013 Elsevier Inc. All rights reserved.