Atom superposition and electron delocalization molecular orbital (ASED-MO) calculations with cluster models of stepped and rough(lll) Pt and Pt-Sn alloy surfaces support a picture wherein the easily-oxidized CO(ads), that is present at high coverges and has an oxidation onset potential 0.4-0.5 V (H-2) lower than the main, hard-to-oxidize CO(ads) peak, goes by a mechanism wherein a H2O molecule attacks the CO. This is followed by oxidation and deprotonation. Results indicate that this reaction is possible at step edge sites of rough surfaces and at edges of substitutional surface islands of two or more Sn atoms in alloy surfaces. The potential dependencies for these reactions as calculated using the ASED-MO band-shift include decreasing activation energies with increasing potential and the prediction of a lower oxidation onset voltage for the alloy compared to the rough surface, in agreement with experimental measurements from the literature. A subsurface Sn layer is calculated to greatly weaken the H2O but not the CO adsorption bond strengths to surface Pt and therefore such sites would not be expected to contribute to the prewave.