We present and discuss sum-frequency vibrational spectra of CO adsorbed on (111)- and (110)-oriented Pt single-crystal electrodes in a 0.1 M HClO4 aqueous electrolyte. For potentials slightly above the potential of hydrogen evolution, CO adsorbs on terminal and 3-fold hollow sites of the Pt(111) surface with frequencies of 2066 cm(-1) and 1788 cm(-1), respectively. Around 0.4 V versus RHE, a structural transition of the CO adlayer takes place with the spectroscopic signature of terminal and bridge-like coordinations similar to 2065 cm(-1) and 1850 cm(-1). respectively). Only terminal adsorption is observed for the (110) surface. Although these frequencies of the CO stretching vibrations are in good agreement with previous infrared reflection-absorption studies, our sum-frequency generation (SFG) spectra indicate a complete oxidation and removal of the CO adlayer at potentials of similar to 0.55 V for Pt(111) and similar to 0.45 V for Pt(110), well below the respective main oxidation potentials in cyclic voltammetry. We show that the lower oxidation potentials observed in SFG are specific for a GO-depleted thin-layer electrolyte and that the CO depletion results from the combination of CO oxidation in the layer and limited diffusion of CO from the bulk electrolyte. We do not find indications for CO adsorbed in a state which is invisible to SFG, as reported in a recent study of CO adsorption on Pt(111) in 0.5 M H2SO4 (S. Baldelli et al. J. Phys. Chem. B 1999, 103, 8920.