Abstract The IR cross section and Stark tuning rate of CO on Pt(111) versus coverage are measured. (The Stark tuning rate is the variation of vibrational frequency with applied electric field.) We interpret the observations as coverage-dependent screening of applied static and IR fields at CO's adsorption site. In contrast to the standard dipole—dipole coupling model, we find that static and IR screening depend differently on coverage. We also find that the local field Stark tuning rate for CO on Pt(111) in vacuum is half that at the electrochemical double layer. Possible explanations of our results are discussed.