CO electro-oxidation on Pt-based electrodes is simulated by density functional calculations on cluster models. A surface bound complex of CO with OH is identified as a crucial intermediate in the mechanism of CO oxidation. The complex under the influence of an appropriate potential and in the presence of water undergoes CO oxidation. The potential energy surface for this reaction as a function of the potential of the electrode is presented, and the barrier for the reaction is shown to vanish above a sufficiently positive applied potential. In addition, the different major barriers for the process are highlighted by comparing the mechanism on pure Pt to a Ru-Pt alloy. (C) 2003 Elsevier B.V. All rights reserved.