In an effort to determine the active state of supported palladium for the direct formation of H2O2 from H-2 and O-2, the catalytic behavior of Pd-0/SiO2, PdO/SiO2 and partially reduced PdO/SiO2 was determined. The results obtained in an ethanol slurry, with chloride ions and H2SO4 being present, showed that the PdO/SiO2 catalyst was almost completely inactive for the formation of H2O2 at 10 degrees C. The Pd-0/SiO2 catalyst exhibited the highest activity for H2O2 formation, and the PdO/SiO2 material, reduced under very mild conditions, exhibited an intermediate activity. The state of Pd on the three catalysts was characterized by XRD, TEM and XPS methods. Only Pd-0 (the metal phase) and PdO were observed on Pd-0/SiO2 and PdO/SiO2, respectively. As expected, with the partially reduced PdO/SiO2 catalyst, both Pd-0 and PdO phases were evident. The TEM results revealed that the Pd-0 particles decorated the larger PdO particles. The results reported here support the role of metallic palladium, rather than the oxide, as the active phase for the direct formation of H2O2.