The influence of protons (mineral acid) and different halide anions (F-, Cl-, Br-, or I-) in the aqueous reaction medium on H2O2 formation in the direct oxidation of H-2 to H2O2 (at 27 degrees C) over different supported Pd catalysts (i.e., Pd supported on carbon, alumina, gallia, silica, zirconia, or ceria) in their reduced form, and also on the activity of the catalysts for the destruction of H2O2 by its decomposition and/or hydrogenation reactions at the conditions close to those used in H-2 to H2O2 oxidation, was thoroughly investigated to gain insight into the roles of protons and halide promoters. Among the halide anions, only the chloride and bromide anions (particularly for the Pd/C and metal oxide-supported Pd, respectively), when added to acidic reaction medium, act as a catalyst promoter to drastically enhance H2O2 formation selectivity or H2O2 yield. But the presence of protons is crucial to this promoting action. The promoting action of the halides is attributed mainly to the large decrease in the H2O2 decomposition and hydrogenation activities of the catalyst resulting from its poisoning of the two consecutive H2O2 destruction reactions. It also may be due to the inhibition caused by the halide promoters for water formation directly from H-2 oxidation. The cations associated with the halide anions play only a minor role or no role in controlling the conversion or selectivity in H-2 to H2O2 oxidation and H2O2 conversion reactions. A plausible mechanism for H-2 to H2O2 oxidation in the presence of chloride or bromide anions in acidic aqueous medium, explaining the beneficial roles of the halide anions and protons for promoting the selective oxidation reaction, is proposed. (c) 2005 Elsevier Inc. All rights reserved.