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The direct synthesis of H2O2from O2and H2provides a green pathway to produce H2O2, a popular industrial oxidant. Here, we theoretically investigate the effects of Pd oxidation states, coordination environments, and particle sizes on primary H2O2selectivities, assessed by calculating the ratio of rate constants for the formation of H2O2(viaOOH* reduction; kO–H) and the decomposition of OOH* (viaO–O cleavage; kO–O). For Pd metals, the kO–H/kO–Oratio decreased from 10−4for Pd(111) to 10−10for the Pd13cluster at 300 K, indicating poorer H2O2selectivity as Pd particle size decreases and low primary selectivities for H2O2overall. As the oxygen chemical potential increases and metals form surface and bulk oxides, the perturbation of Pd–Pd ensemble sites by lattice O atoms results in selectivities that become dramatically higher than unity. For instance, at 300 K, the kO–H/kO–Oratio increases significantly from 10−4to 109to 1016as Pd(111) oxidizes to Pd5O4/Pd(111) and to PdO(100), respectively. In contrast, such selectivity enhancements are not observed for surface and bulk oxides that persistently contain rows of more metallic, undercoordinated Pd–Pd ensemble sites, such as PdO(101)/Pd(100) and PdO(101). These Pd–Pd ensembles are also absent when smaller Pd nanoparticles fully oxidize, indicating that smaller PdO clusters can be more selective for H2O2synthesis. These trends for primary H2O2selectivities were found to inversely correlate with trends for H2O2decomposition rates viaO–O bond cleavage, demonstrating that catalysts with high primary H2O2selectivity can also hinder H2O2decomposition. Ab initiothermodynamic calculations are used to estimate the thermodynamically favored phase among Pd, PdO/Pd and PdO in O2, H2O2/H2O, and O2/H2environments. These results are combined to show that smaller Pd nanoparticles are more prone to be oxidized at lower oxygen chemical potentials, upon which they become more selective than larger Pd particles for H2O2synthesis.