The use of operando FTIR indicates that the high efficiency for NO2 removal from ambient air of iron-containing zeolites comes from their ability to condensate nitric acid. The NO2 removal performance of Fe-modified H-BEA and H-ZSM-5 has been investigated in detail by means of in situ and operando FTIR spectroscopy. The surface characterization using NO2 and NO as probe molecules revealed important contributions of redox processes involving Fe3+ -OH and/or alpha-oxygen species as well as Fe2+ -NO species. In these anhydrous conditions, the NO2 storage performance is mainly due to the disproportion of NO2 leading to NO+ and nitrate species. However, under flow and in the presence of humidity, and thus in more realistic conditions, nitrates and NO+ formation are suppressed. The main mechanism responsible for the wet NO2 removal consists in the formation of both adsorbed nitric acid and gaseous NO. According to the proposed mechanism, a strong positive water-effect is established: the total suppression of NO2 in the presence of humidity together with the formation of NO, which is less toxic than NO2 , makes the investigated zeolites promising candidates for efficient materials used in indoor air treatment. PUBLICATION ABSTRACT