A recent theoretical study of the electrooxidation of carbon monoxide on platinum found a low energy pathway wherein CO(ads) and OH(ads) bond to form COOH(ads) which reacts with a second OH(ads) to yield CO2 and H2O(ads) [J. Electrochem. Soc. 149 (2002) E383]. In the present paper the B3LYP hybrid density functional theory is used to examine an alternative mechanism for COOH(ads) formation and its decomposition. It is found that the direct attack of CO(ads) by H2O to form, by electrooxidation, COOH(ads)+H+(aq) or by H2O(ads) to form COOH(ads)+H(ads) have respective activation energies of 1.7 eV at the calculated reversible potential (0.7 V on the hydrogen scale) or 1.2 eV. Both are higher than that obtained in the above-mentioned study. An activation energy of 0.7 eV calculated for the decomposition of COOH(ads) to CO2 and H(ads) is also greater than that obtained in the earlier study. From this, the reaction mechanism obtained in the earlier study is confirmed as the lowest energy pathway, and the alternative mechanisms explored in the present study are not the cause of the 0.3 V CO(ads) oxidation prewave in acid electrolytes. The over-all conclusion is that the electrode potential-dependent rate-limiting step is the oxidation of water to form the OH(ads) oxidant for CO(ads) removal and the prewave is likely to be due to the lower potentials of OH(ads) formation at step and defect sites where OH adsorption is stronger than on terraces. (C) 2003 Elsevier Science B.V. All rights reserved.