The direct formation of H2O2 from H-2 and O-2 was carried out over a Pd/SiO2 catalyst in a medium of ethanol or water acidified with either H2SO4 or HCl. The H2SO4/ethanol system is the most favorable for peroxide formation. Both the proton and the anion, in the case of Cl-, promote the net formation of the peroxide. Protons inhibit the reduction of H2O2 by H2, and chloride ions limit the direct reduction of O-2 to water, presumably by blocking Pd ensembles. Sulfate ions, being noncoordinating ligands, do not serve this function; therefore the H2SO4/water system is a poor medium for producing the peroxide. By contrast, the H2SO4/ethanol system is believed to be effective because in the presence of O-2, acetate ions are formed from ethanol, and these ions block Pd ensembles in the same manner as chloride ions. (c) 2004 Elsevier Inc. All rights reserved.