Experiments using FT-IR or combined XRD/XAS, both with simultaneous gas phase analysis by MS and chemiluminescence were performed to study reactions over model Pt/Ba/Al2O3 NOx storage and reduction catalysts (NSR). Particular attention was given to the interaction of SO2 with the catalyst components and how these influenced the NO, release and reduction stages. Propene was employed as reductant in gas mixtures with/ without SO2. When catalyst was exposed to SO2 in air, bulk BaSO4 formation occurred readily but without eliminating all of the No, storage capacity. When SO2 was exposed to catalysts in competition with NO2, no bulk sulphate was detected but a diminished NOx capacity was measured suggesting the presence of competing adsorbed SOx species. The latter, in the presence of reductant at higher temperatures, led to rapid displacement of stored NOx and inhibited carbonate formation. (c) 2005 Elsevier B.V. All rights reserved.