The mechanism of NOx adsorption and reaction on a typical 1 wt.% Pt/20 wt.% BaO/Al2O3 NOx storage reduction (NSR) catalyst was studied by the use of in situ Fourier Transform InfraRed (FTIR) spectroscopy. Parallel studies were also conducted with the individual components of the NSR system (i.e., 1 wt.% Pt/Al2O3 and 20 wt.% BaO/Al2O3) in an effort to identify the role and importance of each component in the NSR process. Finally, the role Of SO2 was also investigated, since it has been reported that it represents a poison for the NSR process. The results indicate that at 350 degreesC NOx is stored on the NSR catalyst primarily in the form of barium nitrite and nitrate species. In the presence of Pt a conversion of the surface barium nitrite to bulk barium nitrate is taking place during the storage phase. This step is accelerated in the presence of gas phase oxygen and at elevated temperatures. Both barium nitrite and nitrate species are reduced in the presence of propylene, although at different rates. These reactions are catalyzed by Pt and do not proceed at any significant rate over the BaO/Al2O3 sample. Finally, the formation of surface and bulk barium sulfates was observed when SO2 was introduced as a component of the NO-contaiming gas phase mixture. These species are stable on the catalyst surface under the reduction conditions studied and inhibit the NOx storage process. (C) 2004 Elsevier B.V. All rights reserved.