The effect of sulphur on the C3H6-NO-O-2 and C3H8-NO-O-2 reactions over Pt/Al2O3 was investigated and the results interpreted in terms of previously proposed mechanisms. Ex-situ treatment of the catalyst with SO2 under oxidising conditions, results in the formation of sulphate species on the Al2O3 but not on the Pt. This results in strong poisoning of the NOX reduction activity in the C3H8-NO-O-2 reaction, where NOX reduction occurs on the Al2O3, but has no effect on the C3H6-NO-O-2 reaction, where the reaction occurs exclusively on the Pt surface. These sulphate species were characterised by IR and TPR. Introduction of SO2 (0-100 ppm) into the reaction feed inhibits the C3H6-NO-O-2 reaction, but results in only modest shifts in the C3H6 lightoff curve and the temperature of maximum NOX conversion. The system responds slowly when the concentration of SO2 is changed, but activity returns to its original level when SO2 is removed from the feed.