Summary This API-sponsored experimental program provided data on three critical factors affecting automotive sulfate emissions: gasoline sulfur level@ catalyst system and testing mode. The program was designed to study gasoline sulfur effect@ the other factors were by-products. Four catalyst systems were tested on fuels containing 24 to 469 ppm sulfur using the EPA Sulfate Emission Test (SET)@ a 64 km/h cruise@ and 1975 Federal Test Procedure. Under all test conditions@ SO4= emissions were found to be a linear function of gasoline sulfur content@ though in some cases SO4= emission differences between catalyst systems were larger than the effect of changes in fuel sulfur content for a given catalyst system. SO4= emissions as a function of catalyst system type.