An overall rate expression for the formation of hydrogen peroxide by direct combination (DC) of H-2 and O-2 is required for a rational design of a microreactor and for the determination of the optimum operating conditions for this process. Since the formation of H2O2 by the DC process involves four simultaneous reactions (synthesis of H2O2, synthesis of water, decomposition of H2O2, and reduction of H2O2 by HA, the overall rate expression must take into account the rate expressions of these reactions. In this work, we investigate the mechanism and the rate expression for reduction of H2O2 by H-2. Kinetic experiments were carried out to measure the effects of H2O2 concentration and H-2 partial pressure on the rate of reaction. The reaction was shown to be free of mass transfer limitations at the conditions of the kinetic experiments. A Langmuir-Hinshelwood mechanism for the reduction of H2O2 over Pd/SiO2 Catalyst Was adopted since the simple power-law expression could not explain the obtained kinetic data. A rate expression based on this mechanism was shown to accurately correlate experimental data over a wide range of H2O2 concentration. H-2 partial pressure, and temperature. (C) 2008 Elsevier B.V. All rights reserved.