Adsorption of NO, NO/O-2, and NO2 on Pt-Ba-Al-O system is investigated at 350 degreesC 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 NO2 over Pt in the presence of oxygen. NO2 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 NOx storage from NO/O-2 mixtures. (C) 2003 Elsevier Inc. All rights reserved.