The direct production of H2O2 from H-2 and O-2 (O-2/H-2 = 2) at 25degreesC and 760 Torr occurs in an aqueous phase over colloidal palladium that may be introduced either via PdCl2 or via Pd supported on silica gel (Pd/SiO2). In the latter case, aqueous HCl facilitates the dissolution of the supported Pd. The presence of colloidal palladium was confirmed by electron microscopy. When the solution was either 0.1 M or 1.0 M in HCl, removal of the silica, along with any remaining supported Pd, did not affect the rate of H2O2 formation because the amount of active colloidal Pd remained unchanged. The specific activity of the supported Pd is only 3% of that for colloidal Pd, probably because of transport limitations within the pores of the silica.