Sulfate emissions from two oxidation catalyst equipped, 400 CID vehicles were measured using a dilution tube. The effects of spark timing, exhaust gas recirculation (EGR), oxygen concentration, and catalyst composition were studied. The vehicles were identical Ford LTD's; one was calibrated for 1976 Federal emission standards, the other for the stricter 1976 California standards.At steady-speed 72 km/hr (45 mph), the conversion of SO2 to SO3 over Engelhard IIB catalysts as a function of oxygen concentration is similar to that observed in laboratory studies; a dramatic reduction in sulfate emissions occurs at less than 1% O2. On the EPA Congested Freeway Driving Schedule (CFDS), formerly the Sulfate Emission Test #7 (SET-7), the sulfate emission level is considerably lower than on the 72 kph cruise, and no sharp reduction at low average oxygen concentrations is seen. Sulfate storage in the catalyst washcoat, and subsequent release, is cited to explain these observations. Changing the spark timing from 6 to 16° BTDC and disabling the EGR had no effect on sulfate emissions over the CFDS.Three different noble-metal catalyst formulations were studied: platinum/palladium, platinum/rhodium, and pure palladium, all prepared by Engelhard Industries on Corning substrates. Under steady-speed operation of the Federal vehicle at 72 kph, the emission rates from the catalysts in the order listed above were 47, 17, and 6 mg/km. The corresponding SO2 to SO3 conversions compare well with laboratory experiments. However, quite different results were obtained from the CFDS: for all the catalysts, the emission rates were 4-7 mg/km on the Federal vehicle and 16-28 mg/km on the California vehicle. The mitigation of the differences between catalysts indicates that under realistic driving conditions, the noble-metal catalyst composition may be unimportant in determining sulfate emissions. Morever, misleading conclusions may be derived from laboratory studies which do not incorporate cyclic operation. It is also of note that the higher emission levels from the California car imply that efforts to meet stricter gaseous emission standards may lead to higher sulfate emissions.