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A study has been made of potential methods for controlling SO4= emissions from oxidation catalyst-equipped vehicles. The methods considered included operating condition and catalyst changes, as well as the use of a vehicle trap for SO4=. Emissions of SO4= from non-catalyst cars were also measured. The only engine operating variable we found to significantly lower SO4= emission was exhaust gas O2 level. Limiting air pump use reduced SO4= emissions by factors of 5 to 7 over the FTP, and by factors of 2 to more than 10 at 96 km/h. Some increase in CO and HC emissions was observed when the greatest SO4= reductions were achieved, but it appears that properly modulated carburetion could overcome this problem. Limited excess air shows great promise as a means of minimizing SO4= emissions.