Synthesis gas (a mixture of CO and H-2) oxidation is studied over a supported Pt/Al2O3 catalyst in a novel microreactor fabricated for studying the intrinsic chemical kinetics of highly exothermic reactions. CO was found to significantly inhibit H-2 oxidation. In contrast, H-2 addition promotes CO oxidation at low mole fractions but has a small promoting effect at high hydrogen mole fractions. As a result, the apparent reaction order of H-2 changes from positive to zero. The change in hydrogen reaction order is associated with hysteresis. Possible mechanisms for the observed behavior are discussed. (C) 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.