The carbon monoxide layer on a platinum electrode, which is adsorbed at 0.05 V relative to a normal hydrogen electrode (NHE) in 0.5 M sulfuric acid, and its oxidation to carbon dioxide at higher electrode potentials have been studied by electrochemical and in situ Fourier transform infrared reflection-absorption spectroscopy (FT-IRRAS). Polarization-modulated FT-IRRAS was used to measure the vibrational spectra of adsorbed carbon monoxide as well as the evolved CO2 as a function of electrode potential. It is shown that the dominant surface species is linearly adsorbed CO but that the bridge-bonded species is oxidized first at about 0.20 V, giving rise to a decrease in the linear C-O stretching frequency along with a broadening of the band. Oxidation of the linearly adsorbed CO begins at 0.35 V, producing a further, sharp decrease in the C-O stretching frequency as well as a considerable broadening of the band. It is concluded that the oxidation of the CO adlayer produced at 0.05 V occurs randomly throughout the layer, and not on the edges, which is characteristic of CO adsorbed at 0.4 V. It is proposed that the difference in behavior of these two kinds of adsorbed CO is due to crystallographic modification of the platinum surface when the CO is adsorbed at 0.05 V in the hydrogen region resulting in a higher density of bridge-bonded CO. © 1986, American Chemical Society. All rights reserved.