Carbon-supported platinum (Pt/C) with an adsorbed layer of underpotential deposited (upd) Sn is a much better catalyst for the methanol oxidation reaction (MOR) than a carbon-supported platinum-tin (PtSn/C) alloy. In situ X-ray absorption (XAS) was used to determine the differences in the effects that the two methods of Sn addition have on the electronic properties and the structural properties of the catalyst. X-ray diffraction and XAS at the Pt L-3 and L-2 edges indicate that the PtSn/C catalyst has a Pt3Sn L1(2) structure, and alloying with Sn causes partial filling of the Pt d band vacancies and an increase in the Pt-Pt bond distance from 2.77 to 2.8 Angstrom. However, upd Sn does not perturb Pt structurally or electronically XAS at the Sn K edge indicates that both the upd Sn on Pt/C and the surface Sn on PtSn/C are associated with oxygenated species at all potentials, and that the nature and strength of the Sn-O bonds are potential dependent. The differences in the activity of the two catalysts for the MOR are due to the effects of allaying on the Pt electronic structure that inhibit the ability of the Pt to adsorb methanol and dissociate C-H bonds. The ability of PtSn/C to adsorb oxygen at low potentials enhances its activity for CO oxidation.