This paper reports the oxidation reactivities of three hydrocarbons over an aged palladium (Pd) catalyst during simulated cold-start conditions. Local reaction rates and species concentration profiles on a catalyst of Pd on Al2O3 with ceria are reported for A/F ratios from 14 to 15 at a temperature of 300°C. The synthetic exhausts contain 1-butene (C4H8) as a surrogate for the most reactive hydrocarbons, and C3H8 and CH4 as surrogates for species with intermediate and very low reactivities, respectively. All gas mixtures contain representative amounts of CO, CO2,O2, H2, and H2O in N2.The results demonstrate that the control system on our catalytic flow reactor maintains the desired temperature to within 3°C at the highest conversions ever encountered with auto exhausts, and that our product analysis scheme monitors local reactivities of all species simultaneously, regardless of the conversion level. Its performance with multicomponent exhaust mixtures shows distinct advantages over both differential and integral reactors. In addition, the data reveal several interesting features of multicomponent fuel oxidation under excessively fuel rich conditions. First, the oxidation of CO and hydrocarbons over an aged Pd-catalyst follows similar conversion histories for excessively rich and near-stoichiometric O2 levels. Second, CO is the most reactive species on supported Pd, showing both the highest overall conversion and the highest local reactivities at all observation points. Carbon monoxide also progresses further than any other species along the cycle from kinetic control to diffusion-limited oxidation, although we did not observe a surface concentration of zero even for CO. Third, the hydrocarbons oxidize simultaneously, but at rates that follow the ranking of reactivities seen in tests with individual fuel species. Whereas the magnitude of the C4H8 conversion rate is much lower than CO's because of its much lower inlet level, this hydrocarbon is eliminated on nearly the same time scale as CO. But C3 H8 and CH4 are converted on much longer time scales.