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 H 2 O 2 by the DC process involves four simultaneous reactions (synthesis of H 2 O 2 , synthesis of water, decomposition of H 2 O 2 , and reduction of H 2 O 2 by H 2 ), 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 H 2 O 2 by H 2 . Kinetic experiments were carried out to measure the effects of H 2 O 2 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 H 2 O 2 over Pd/SiO 2 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 H 2 O 2 concentration, H 2 partial pressure, and temperature.