The vibrational spectrum of carbon monoxide at the Pt/acetonitrile interface as a function of applied potential has been investigated using the surface-enhanced Raman spectroscopy technique. The electrolyte is 0.1 M LiClO4. The bands observed at ca. 506 and 2055 cm-1 are attributed to the platinum−CO (νPt-C) and intramolecular C−O (νC-O) stretching vibrations, respectively, suggesting linearly adsorbed CO on platinum. The νPt-C band has an experimental Stark tuning rate of −4 cm-1/V, while the slope of νC-O band frequency versus potential approaches zero before the onset of electro-oxidation of COads, possibly resulting from low surface coverage of CO on platinum. The roughened Pt electrode surface has a high electrocatalytic activity, on which CO electro-oxidation occurs at ca. −0.7 V (vs Ag/Ag+). The main product of COads oxidation is confirmed to be carbonate due to the existence of trace water in the double-layer region as a source of oxygen for the reaction. It has also been found that the solvent ac...