B3LYP hybrid density functional calculations show that CO and OH bonded to Pt-2 react with a low activation energy (calculated 0.44 eV) to form - COOH bonded to a onefold site of Pt-2. An OH bonded to the adjacent onefold site attracts the acid group by hydrogen bonding and a low activation energy (calculated 0.23 eV) separates this complex from CO2 + H2O -Pt-2. The similarity of the bond strengths between Pt-2 and CO, OH, and H2O to measured chemisorption bond strengths from the literature makes these results worth considering as a model for the electro-oxidation of CO(ads) by OH(ads) on platinum anodes. Calculations of activation energies for the oxidative deprotonation of Pt-2 - COOH indicate that high activation energies are to be expected for the reaction -COOH(ads) + H2O --> CO2 + H3O+(aq) + e(-)(U) at the electrode potentials where CO(ads) is removed (similar to0.6 V). Therefore, the formation of OH(ads) from oxidation of H2O, shown in another study to have a low activation energy and a reversible potential of similar to0.57 V, is concluded to be the cause of the observed overpotential for the electro-oxidation of CO(ads).