A combined in situ spectroelectrochemical and theoretical study of CO adsorbed on clean and palladium-covered Pt(111) single-crystal electrodes is presented. The in situ spectra of CO adsorbed on the Pt(111) and Pd supported on Pt(111) single-crystal electrodes show different bands corresponding to the C-O stretching mode that are assigned with the help of density functional cluster model calculations. For the palladium-covered Pt(111) electrode with a palladium coverage close to the monolayer, the resulting spectra can be interpreted by using a pure Pd cluster model only. A shift to higher vibrational frequencies is observed which increases with increasing CO coverage. This shift can be interpreted as a change in the adsorption site because of coverage effects in agreement with UHV results at 300 K. The present study shows that the combined use of spectroelectrochemical and first principles calculations is a promising and powerful tool for the interpretation of complex electrochemical interfaces.