The direct formation of H2O2 from H-2 and O-2 is an attractive alternative to the current technology that involves the hydrogenation of an alkylanthroquinone to the corresponding hydroquinone, followed by the reaction of the hydroquinone with oxygen to yield H2O2. The direct reaction is generally catalyzed by palladium, although there is evidence that the addition of small amounts of platinum may enhance the yield of peroxide. The palladium may be in a supported form or it may be present as a colloid derived from PdCl42- ions in solution. The reaction takes place in a three-phase system that includes the reagent gases, the palladium catalyst, and a solvent, which is usually water. Other solvents, including methanol, are also effective. The solutions contain an inorganic acid and often halide ions (Cl- or Br-) to inhibit the decomposition of H2O2 and perhaps the nonselective oxidation of H-2 to H2O. At sufficiently high concentrations of an acid, the dissolution of supported palladium occurs. Because of the three-phase system, the rates may be transport-limited. The solubility of H-2 in H2O is small; therefore, high reagent pressures are used to enhance formation rates and the ultimate yields of H2O2. Safety issues that result from having the reagents at high pressures are significant, and to avoid contact of H-2 and O-2 in the gas phase, the introduction of H-2 through a membrane has been demonstrated. A number of fundamental issues remain unresolved including the active state of the palladium and the reaction mechanism, although it has been shown that the oxygen remains in a diatomic form during the production of H2O2. (C) 2003 Elsevier Science (USA). All rights reserved.