This paper reports the results of a study of the combined oxidation of hydrogen and carbon monoxide on a platinum diesel oxidation catalyst. Experimental results for the light-off curves obtained in a monolith supported catalyst are shown for different concentrations. The presence of hydrogen is shown to promote the oxidation of CO, with the largest effect shown for the first small addition of hydrogen, then a progressively decreasing effect as the hydrogen concentration is increased. The enhancement effect can be successfully simulated using a kinetic model in which the activation energy for the desorption of CO is decreased as a function of hydrogen adsorption. Hydrogen is allowed to adsorb on sites only accessible to hydrogen and not CO or oxygen. (C) 2008 Elsevier B.V. All rights reserved.