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The electro-oxidation of CO on model platinum-tin alloy catalysts has been studied by ex-situ electrochemical measurements following the preparation of the Pt(111)/Sn(2 × 2) and Pt(111)/Sn(√3 × √3)R30° surfaces. A surface redox couple, which is associated with the adsorption/desorption of hydroxide on the Sn sites, is observed at 0.28 V[sub RHE]/0.15 V[sub RHE] in H[sub 2]SO[sub 4] electrolyte on both surfaces. Evidence that it is associated with the adsorption of OH comes from ex-situ photoemission measurements, which indicate that the Sn atoms are in a metallic state at potentials below 0.15 V[sub RHE] and an oxidized state at potentials above 0.28 V[sub RHE]. Specific adsorption of sulfate anions is not associated with the surface process since there is no evidence from photoemission of sulfate adsorption, and the same surface couple is observed in the HClO[sub 4] electrolyte. CO is adsorbed from solution at 300 K, with saturation coverages of 0.37 ± 0.05 and 0.2 ± 0.05 ML, respectively. The adsorbed CO is oxidatively stripped at the potential coincident with the adsorption of hydroxide on the tin sites, viz., 0.28 V[sub RHE]. This strong promotional effect is unambiguously associated with the bifunctional mechanism. The Sn-induced activation of water, and promotion of CO electro-oxidation, is sustained as long as the alloy structure remains intact, in the potential range below 0.5 V[sub RHE]. The results are discussed in the light of the requirements for CO-tolerant platinum-based electrodes in hydrogen fuel cell anode catalysts and catalysts for direct methanol electro-oxidation. [ABSTRACT FROM AUTHOR]