Sum frequency generation (SFG) vibrational spectroscopy in combination with cyclic voltammetry (CV) was used to investigate the oxidation of carbon monoxide (CO) in acetonitrile (CH3CN)/water (H2O) solutions at the platinum (Pt) electrode surface. CO oxidation on Pt was studied as a function of water concentration and electrode potential. In electrolyte solutions containing small concentrations of water, a preoxidation wave was observed at +900 mV vs SCE whereby a small amount of the adsorbed CO was oxidized. This preoxidation wave was followed by the main oxidation of CO on Pt at approximately +1700 mV vs SCE. Spectroscopy and voltammetry data in these essentially nonaqueous electrolytes indicate an oxidation intermediate of CO to CO2 that was nearly parallel to the Pt electrode surface.