H2O2 was directly synthesized from the elements over supported I'd membranes in O-2-saturated, aqueous H2SO4 (0.03 mol/l) between 303 and 348 K while feeding H-2 through the membrane. The H2O2 yield increased with temperature despite H2O2 selectivity declining simultaneously due to faster H2O2 decomposition. Both H2O2 yield and selectivity improved with H-2 feed pressure, however. Comparison of H-2 consumption during H2O2 synthesis with the intrinsic membrane H-2 permeability and the energetics of H-2 oxidation on Pd surfaces revealed that the H2O2 productivity gain with temperature was primarily owed to the acceleration of reaction kinetics. The H2O2 selectivity enhancement with H-2 feed pressure is attributed to the inhibition of direct H2O formation as a result of hindered O-2 dissociation due to blocking of O atom adsorption sites on the membrane surface because of increasing occupation by H atoms. H2O2. decomposition and reduction on the Pd membrane surface accelerate much faster with temperature than H2O2 formation and need to be suppressed in order to maximize H2O2 yield and H-2 utilization at higher temperatures. (C) 2009 Elsevier B.V. All rights reserved.