The sulfur dioxide to sulfur trioxide conversion has been measured for three different automotive exhaust catalysts. Two of the catalysts were 1975 production catalysts (Engelhard IIB and Matthey-Bishop 3C) and the third is a palladium catalyst on a monolith support. The carbon monoxide and propylene conversions were also measured so that the activity of the three catalysts for these gases could be compared to their sulfur dioxide activity. The measurements were made using a flow reactor with simulated exhaust gas and show that, while the carbon monoxide and propylene conversions were very similar for all three catalysts, there was a wide range of sulfur dioxide conversions. At 525°C and 73,000 hr-1 space velocity the sulfur dioxide conversion was 70% for the Engelhard IIB, 40% for the Matthey-Bishop 3C and from 25 to 70% for the palladium catalyst. The palladium catalyst has a range of conversions under these conditions which are associated with different states of the catalyst. It is not clear which state prevails in an automobile exhaust system.