The effects of underpotential-deposited lead on the adsorption of CO on Pt(111) surfaces have been investigated in 0.1 M HClO4 by in situ infrared reflection absorption spectroscopy (IRAS). Lead coverages of about 0.4 on Pt(111) electrodes polarized at 0.1 V vs RHE prevent CO from adsorbing on multiply bonded sites, reducing the overall coverage to about a third of that observed in the absence of coadsorbed Pb under otherwise identical conditions, yielding a single, sharply-defined feature in the infrared reflection absorption spectra characteristic of atop-bonded CO. However, the integrated absorption band of atop CO at 0.1 V in the presence of coadsorbed Pb was found to be about 3 times higher than that predicted from the corresponding spectral features observed for Pb-free surfaces at that specific coverage corrected for dipole-dipole coupling interactions. Although much of this effect may be due to a Pb-induced decrease in the dielectric screening within the CO adlayer, the gains in intensity are too large to be explained solely on this basis, pointing to chemical factors, such as Pt-Pb charge polarization, as an important factor.