Direct combination (DC) of hydrogen and oxygen over a heterogeneous catalyst in a microreactor is a novel method of producing hydrogen peroxide with significant economic advantages over the currently dominant anthraquinone autoxidation method. A kinetic rate expression for this reaction is required for design and modeling of a microreactor for the DC process. Since the formation of H2O2 by the DC process involves four simultaneous reactions (synthesis of H2O2, synthesis of water, reduction of H2O2 by H-2 and decomposition of H2O2), the overall rate expression must take into account each of these reactions. In this work, we describe a reactor model that involves the four component reactions as well as mass transfer effects. The model is verified by comparing the predicted reactor performance with experimental data. In addition to providing a tool for reactor design, the model also confirms important assumptions regarding the mechanism of DC reaction. (c) 2009 Elsevier Ltd. All rights reserved.