A polycrystalline platinum electrode covered with various amounts of electrosorbed carbon monoxide was transfered to ultrahigh vacuum for investigation by thermal desorption spectroscopy (TDS). Solution dosing yielded a saturation coverage up to 15% higher than that obtained by gas phase dosing at room temperature. The thermal desorption features for electrosorbed and gas-phase-dosed carbon monoxide are similar for saturated layers. Submonolayer coverages, obtained by partial oxidation as well as by emersion during steady state carbon monoxide electro-oxidation, show thermal desorption features indicating an island structure with high carbon monoxide concentration. No electrolyte residues were found for emersed saturated carbon monoxide layers, but water and sulfate thermal desorption signals increase with decreasing carbon monoxide coverage. Coadsorbed oxygen was found at low carbon monoxide coverages when the electrode was emersed during steady state carbon monoxide electro-oxidation near 900 mV/RHE.