Benzoquinone (BQ) and O-2 are among the most common stoichiometric oxidants in Pd-catalyzed oxidation reactions. The present study provides rare insights into mechanistic differences between BQ and O-2 in their reactivity with a well-defined Pd-hydride complex, Pd(IMes)(2)(H)(O2CPh) (1). BQ promotes the reductive elimination of PhCO2H from 1 and catalyzes the formation of a Pd-II-OOH complex when this reaction is carried out under aerobic conditions. These results have important implications for Pd-catalyzed oxidation reactions.