An atom superposition and electron delocalization molecular orbital (ASED-MO) study shows that tin atoms alloyed substitutionally into platinum electrode surfaces are inactive in generating OH(ads) for the electro-oxidation of the CO(ads) poison generated during the operation of methanol fuel cells. Such tin atoms, though they donate to Pt, are not good accepters for H2O lone-pair donation bond formation because of the way in which their 5p orbitals mix with the Pt valence band. Thus substitutional Sn atoms in the Pt surface do not attract or activate H2O. OH is also found to adsorb weakly to substitutional Sn atoms compared to surface Pt atoms, the opposite of the diatomic SnO and PtO bond strengths. This is because the OH is essentially reduced by neighboring Pt atoms and not the Sn atom to which it is bound. When bound to substitutional Sn atoms, OH is calculated to be relatively active in oxidizing CO(ads) on adjacent Pt atoms, but the inability of the surface to generate such OH implies a different mechanism must be responsible for the electrocatalysis, perhaps involving adsorbed Sn atoms or Sn complexes.