The interaction of SO2 with Pt/BaCO3/Al2O3 NOx storage catalysts under lean and rich conditions in the absence and presence of water was investigated by means of in situ FTIR spectroscopy at 350 degrees C. For the lean condition (SO2 + O-2), surface and bulk barium sulfates were formed in addition to sulfites and surface sulfates on alumina. Pt does not appear to play an essential role in the formation of these species under lean conditions. In contrast, under reducing conditions (SO2 + H-2), Pt catalysed the reduction of SO2 with H-2 to form reduced sulfur species that accumulate on both Pt and barium sites. A subsequent treatment with NO2 resulted in the reduction of NO2 in the oxidation of these sulfur-containing species to form bulk barium sulfate instead of being stored over barium sites. Introducing water significantly affected sulfur trapping over the catalyst surface. During the lean conditions, the presence of water resulted in almost no surface barium sulfate formation. Meanwhile, the formation of bulk barium sulfate was increased, probably due to enhanced sulfate formation. However, the presence of H2O under reducing conditions resulted in significantly lower sulfur storage on both barium and alumina in comparison with humid lean exposure conditions. (c) 2006 Elsevier Inc. All rights reserved.