The vibrational dynamics of CO at two electrochemical interfaces is studied as a function of the electrode potential, for CO/Pt(100) in 0.1M aqueous H 2 SO 4 and CO/Pt(110) in 0.05M C 1 6 H 3 6 C lNO 4 acetonitrile electrolyte. The measured lifetime of ∼1.7 ps is in line with those previously determined for dry CO-metal interfaces. The lifetime appears to be independent of the electrolyte composition, and unaffected by an electrode-potential variation as large as 2 V achieved for the non-aqueous electrolyte experiment. These measurements suggest that the 2π * CO acceptor orbital involved in the substrate/adsorbate charge transfer process is much broader than 0.8 eV.