Among the platinum group metals, it is Pd that provides the highest catalytic activity for the complete oxidation of hydrocarbons in automotive exhausts (particularly for gasoline engine applications) and the combustion of methane in gas-powered turbines.For diesel emissions, palladium is commonly used in combination with platinum to catalyze oxidation reactions. Together these two metals exhibit a synergistic relationship and are able to oxidize CO, hydrocarbons and NO produced by diesel engines while demonstrating remarkable hydrothermal stability not seen for Pt-only catalysts. With the push towards higher efficiency engines, there is a need to achieve conversion of hydrocarbons at lower temperatures. Hence, this study focuses on the hydrocarbon oxidation performance of palladium catalysts which are known to be very active for this reaction. The nature of the active sites is still a matter of debate and it has been suggested that the active sites could be attributed to bulk PdO, metallic Pd, a surface Pd oxide on metallic Pd,a PdO-rich surface,or co-existence of Pd and PdO. In this study we set out to reexamine the roles and relative reactivity of metallic Pd and PdO since this will help in the design and operation of low temperature catalysts for emission control. The contradictory assignments for active sites of Pd catalysts are due, in part, to the facile and reversible conversion between Pd metal and PdO and because observations are often made under very different experimental conditions, especially in the case of methane oxidation. Above the decomposition temperature of PdO (which depends on the prevailing oxygen pressure), the only observed phase is Pd metal. However, at the high temperatures encountered the kinetics are so rapid that both phases are found to be active for methane oxidation. This is because Pd metal exposed to oxygen at high temperatures forms a strongly bound surface oxide that passivates the metal and prevents the formation of bulk oxide. It was shown that an unreactive surface oxide was formed on Pd when exposed to ~700 °C at high oxygen pressures. The co-existence of Pd and PdO represents a metastable state due to the presence of passivated Pd metal which resists oxidation. Our previous work on catalysts that were quenched in liquid nitrogen from high temperatures shows that Pd and PdO can co-exist, but as separate phases. If PdO formed as a shell on the surface of Pd metal, we would not expect to see reactivity spanning that of metallic Pd and PdO. In this work, we follow industry-standard light off protocols to compare the performance of pre-reduced Pd and fully oxidized PdO. We also used in-situ X-ray absorption near edge spectroscopy (XANES), and temperature programmed reduction and oxidation (TPR/TPO) to correlate the reactivity with catalyst structure. The comparison of reactivity of metallic Pd and PdO provides insight into the factors responsible for low temperature hydrocarbon oxidation activity.