It has been observed in this work and previous studies that Pt3Sn alloy surfaces are very effective catalysts for CO electrooxidation, but not for methanol electrooxidation. Since COads is postulated to be an intermediate in methanol electrooxidation on Pt alloy surfaces, the relative inactivity of Pt3Sn for methanol oxidation appears paradoxical. We present an explanation for this apparent contradiction diction in terms of a unique state of COads on this surface, which is not the same state of CO as occurs on either Pt-Ru or pure Pt surfaces. It is also not a state of COads which is produced by methanol dehydrogenation. The state is unique in the sense that a significant fraction of COads is oxidized at a much lower (<400 mV) potential than the rest of the COads, a phenomenon that does not occur on any other Pt and Pt-alloy surfaces examined in the same way. This CO state is only formed at high coverages by direct adsorption from dissolved CO and is not formed by the dehydrogenation of methanol, since the multiple Pt atom sites needed to dehydrogenate methanol are blocked by COads at low coverage.