Recent reports of significant H2S emissions from single-bed three-way catalyst equipped vehicles have prompted vehicle and laboratory studies of the mechanism of H2S formation over three-way catalysts. Vehicle studies have shown the occurrence of short duration H2S emissions significantly higher than fuel sulfur inlet levels under certain transient conditions. These emissions cannot be explained by the previously reported steady-state conversion of SO2 to H2S. Laboratory flow reactor studies have identified a second mechanism for H2S formation involving the rapid reductive release of sulfur stored on the catalyst surface. This mechanism can account for H2S emission levels which can significantly exceed inlet fuel sulfur levels for short durations. Results are presented to define factors which affect H2S formation under these two mechanisms and to determine methods for controlling these emissions under vehicle operation.