Abstract Adsorption of NO, NO/O 2, and NO 2 on Pt–Ba–Al–O system is investigated at 350 °C by the transient response method (TRM) and FT-IR spectroscopy. The data suggest that in the presence of oxygen, NO is effectively adsorbed (at a Ba site in proximity of a Pt site) through stepwise oxidation to form at first nitrites that are progressively transformed into nitrates. NO is also oxidized to NO 2 over Pt in the presence of oxygen. NO 2 is directly adsorbed to form Ba nitrates according to a disproportionation reaction, which occurs with the evolution of NO. The stepwise oxidation route seems to play a major role in NO x storage from NO/O 2 mixtures.