The formation of species when BaSnO3 is exposed to SO2 under a variety of conditions is reported and discussed. Direct contact with 25 ppm SO2 during 15 cycles of Lean-Burn operation leads to the complete loss of NO, absorption capacity owing to the formation of stable and irreversible bulk-like sulphates identified by FT-IR (990, 1060, 1125 and 1190 cm(-1)) and TGA-MS. When both NO2 and SO2 are present, the competition between the formation of nitrate and sulphate favours the latter, leading eventually to deactivation. If the catalyst is pre-nitrated before exposure to SO2, the co-existence of nitrates and sulphates between 100 and 300 degreesC is possible and improved sulphur tolerance is obtained. (C) 2001 Elsevier Science B.V. All rights reserved.