A microkinetic model is developed to simulate the promoting effect of H-2 on the light-off of CO oxidation on platinum. It is shown that the coupling of the CO and the H-2 oxidation sub-mechanisms, via the hydroxyl intermediate and its reaction with CO to carboxyl, is crucial to describe the process. This carboxyl-path was not considered in previous modeling studies. (C) 2011 Elsevier B.V. All rights reserved.