The direct synthesis of hydrogen peroxide from hydrogen and oxygen was investigated using capillary and stacked microreactors in which the silicone wall of a microchannel had been coated with Pd/C catalyst. To make the H2O2 production system compact, a flow-through solid polymer electrolyte (SPE) electrolyzer, which was used for the H-2- and O-2-feeds instead of gas cylinders, was connected to the microreactor. The influence of the gas and liquid flow rates and of the reaction temperature on H2O2 production was investigated to identify the optimal operating conditions for the stacked microreactor. In the previous study [Kusakabe, K., Kawaguchi, K., Maehara, S. and Taneda, M., 2007, Direct formation of hydrogen peroxide over palladium catalyst based on water electrolysis, J Chem EngJpn, 40: 523-528], direct synthesis of H2O2 over Pd/C catalyst was performed using a slurry batch reactor with two sheets of electrode for H-2 and O-2 evolution. The maximum H2O2 concentration of 3.4 x 10(-3) mol/L was attained after a 3-h reaction. In this study, a H2O2 concentration of 8.3 x 10(-3) mol/L was obtained at 10 degrees C in a 0.1 mol/L solution of HCl at the residence time of 93s in the stacked microreactor. In addition, H2O2 was successfully produced when pure water was used as the reaction liquid with H2 and 02 in the stacked microreactor. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.