Additional evidence is presented for the catalytic role of aqueous colloidal palladium in the direct conversion of H-2 and O-2 to H2O2. Reactions typically were carried out at an O-2/H-2 ratio of 2, and 25 degreesC, by introducing the gases at 1 atm through a frit into the aqueous slurry. The source of palladium was either PdCl2 or reduced palladium supported on silica gel. During the course of the reaction the palladium is distributed among PdCl42-, colloidal palladium, palladium deposited on the frit, and Pd/SiO2 when SiO2 is present. Although the amount of colloidal palladium differs, depending on its source and the time on stream, the rate of H2O2 formation is proportional to the amount of colloid present at a particular time. Maximum rates were observed for colloids prepared from Pd/SiO2; however, the largest H2O2 concentration of 0.7 wt% was attained when PdCl2 was the source of colloidal palladium. The ultimate steady-state concentration of H2O2 is, in part, affected by the rate of H2O2 decomposition, which is relatively large over the Pd/SiO2 sample that was tested. The rate of decomposition also is determined by the H+ concentration. Results obtained with a mixture of O-16(2)/O-18(2) confirm that oxygen remains undissociated during the formation of H2O2, which may explain why palladium is uniquely suited as a catalyst for this reaction. (C) 2003 Elsevier Science (USA). All fights reserved.