Carbon monoxide adsorbed on a smooth platinum electrode was investigated in the potential range including the double-layer region and the region of its steady-state oxidation in solutions of 1 M HClO4, H2SO4, or HCl by using in situ polarization modulated Fourier transform IR reflection absorption spectroscopy (FT-IRRAS). The intensity, position, and line width of the IR absorption band of the linearly adsorbed CO species were measured as a function of potential. The evolution of carbon dioxide produced by the oxidation of the adsorbed CO was observed in situ by monitoring the IR absorption spectrum of the carbon dioxide. Oxidation of the adsorbed CO is completed before the formation of a surface oxide or an adsorbed oxygen layer. It was found that the linear increase of the CO stretching frequency continued even into the CO oxidation potential region where the coverage of CO decreases appreciably. It is concluded that oxidation proceeds mostly at the edges of CO islands where the water molecules are adsorbed adjacent to the CO sites. The band due to CO adsorbed in the bridged form was observed in 1 M HC104 by electrochemically modulated IR reflection spectroscopy (EMIRS), and it was concluded from its potential dependence that the bridged CO oscillator strength decreased at more positive potentials. © 1985, American Chemical Society. All rights reserved.