The effect of small amounts of sulphur dioxide on the performance of a three-way vehicle exhaust catalyst has been studied. Removal of propane from a stoichiometric gas mixture containing oxygen was reduced, but the effect was largely reversible. Small amounts of SO2 poisoned irreversibly the steam reforming and water-gas shift reactions, apparently as a result of a support sulphate/metal sulphite/sulphide couple under oxidising-reducing conditions.