The formation of H2O2 from H-2 and O-2 and H2O2 decomposition/hydrogenation have been investigated in an acidic aqueous reaction medium over halide-assisted Pd/Ga2O3 catalyst system. The halide ions were introduced in the catalytic system by directly adding the halide ions to the reaction medium or by indirectly through the catalyst. The H2O2 formation selectivity was found to be influenced significantly with the addition of halide ions to the catalytic system. The H2O2 formation and H2O2 decomposition/hydrogenation studies over halide-modified Pd/Ga2O3 catalysts (pretreated under different gaseous atmosphere viz. air and nitrogen) revealed that bulk Pd oxidation state and nature of halide ions incorporated in the catalyst cooperatively promote the selective H2O2 formation. The decomposition and hydrogenation H2O2 on the Pd/Ga2O3 catalyst were also found strongly depend upon the nature of the halide ions introduced in the catalytic system. The selectivity for the conversion of H, to H2O2 was enhanced markedly due to the presence of Br- ions in the catalytic system, irrespective of the bulk palladium oxidation state (PdO or Pd-0) in the catalyst. The beneficial role of Br- ions is attributed to the inhibition of the parallel H-2 to water oxidation and the consecutive H2O2 decomposition and hydrogenation reactions over the catalyst. (c) 2007 Elsevier B.V. All rights reserved.