The relative effectiveness of Pt and Pd in TWC catalysts for converting hydrocarbon (HC) species was investigated using engine dynamometer and vehicle FTP evaluations.An engine-aged Pd/Rh TWC catalyst showed higher HC conversions in Bag-1 than a comparably aged Pt/Rh TWC catalyst even though light-off activity for the Pt/Rh was approximately 40 °C lower than for the Pd/Rh. Analysis of the Bag-1 HC species by capillary gas-chromatography suggested that Pd was more effective in oxidizing both C2-C5 paraffins and aromatic HCs. In Bag-2 and -3, where Pd/Rh HC conversions were lower than those of the Pt/Rh, the Pd/Rh was superior to the Pt/Rh in converting the aromatic HCs.A Pt/Rh at the front (upstream location) and the Pd/Rh at the rear position was found effective for lowering HC emissions in comparison to the Pt/Rh-Pt/Rh converter system while maintaining CO and NOx conversion performances.These results suggest that including Pd in a TWC catalystsystem would be a valuable method toward meeting TLEV and LEV emission regulations.SEVERAL EMISSION control system strategies are being considered in an effort to meet the stringent California hydrocarbon emission regulations. They include (1) catalysts located at a closed coupled manifold position,1) (2) electrically heated catalysts,2), 3), 4) and 5) (3) hydrocarbon traps,6) and (4) exhaust gas ignition. 7) Each adresses lowering HC emissions during the time the exhaust gas temperature is too low for the catalyst in the underfloor position to be activated. However, these system require dramatic changes in emission control devices and will result in cost increases. Therefore, improvement in HC performance of current TWC catalysts is also required to help in lowering these emissions.Recently, it has become important to know the HC species and ratios present in the exhaust under complicated driving conditions, rather than lumping HCs into a total mass emission. This stems from a consideration of the following environmental issues and how they relate to each HCs characteristics and relative catalytic reactivity:8 ), 9 ) and 1 0 ) (1)HCs with high photochemical reactivity (ethylene, formaldehyde, propylene, toluene, xylene) (2)HCs with toxicity to human health (benzene, 1, 3-butadiene, formaldehyde) (3)HCs with low reactivities to catalytic combustion (low molecular weight paraffins, high molecular weight aromatics, etc.) The use of reformulated fuel is being discussed as an approach to lowering both photochemically and toxic HCs. 9 ) 1 1 ) 1 2 ) I n particular, detailed investigations have been made on the effects of reformulated gasoline compositions on HC speciation in engine out and tail pipe emissions as a part of the Auto/Oil Air Quality Improvement Research Program. 13), 14), 15), 16), 17), 18), 19) and 20)* Numbers in parentheses designate refferences at end of paperIt is felt that when reformulated fuels are used, tail pipe HC emissions would be affected by (1)catalyst type, and (2)precious metal species used in the catalyst. A technical report21), 22) and 23) has described the effects of catalyst type on lowering total HC emissions, but it has not referred directly to the reactivity of HCs versus the catalyst type. This had been discussed with respect to precious metal species at the initial stage of autoexhaust catalyst development, but precious metal catalysts supported on pelletted A1203 were used. 24) A study using current high technology TWC catalysts containing the oxygen storage component, Ce02 has not yet been made.In this study, differences in reactivity of HC species toward current Pt/Rh and Pd/Rh TWC catalysts were investigated. The effects of a combined use of Pt/Rh and Pd/Rh to maximize HC reactivity will also be discussed.