It has been reported by Dow Chemical Co. that oxidation catalysts cause increased particulate emissions from automotive exhausts. We find that this particulate consists of aqueous H2SO4 droplets.Current work undertaken between Ford and Battelle Columbus Laboratories, using an engine dynamometer, shows that the fuel sulfur emerges from the engine as SO2. At 60 mph road load, a monolithic oxidation catalyst converts almost half of the SO2 into SO3, the bulk of which is emitted from the tailpipe as H2SO4. The mass median diameter is smaller than 0.25 μm. Some ammonium sulfate is present, but the predominant sulfate species is H2SO4, totalling some 40% of the gross particulate mass depending on humidity. The rest is primarily water, associated with the hygroscopic H2SO4. Without a catalyst, the H2SO4 is <1/50 as much as with catalyst, the bulk of the fuel sulfur being emitted as SO2.