The dependence of adsorbate vibrational frequencies on the surface potential, phi ("Stark-tuning" effect), observed in electrochemical systems is exploited for the same metal surfaces in contact with ambient-pressure gases so to estimate phi values in the latter environment. Saturated CO adlayers on palladium and platinum films are examined along these lines by using surface-enhanced Raman spectroscopy (SERS) to obtain frequencies for both the C-O (nu(CO)) and metal-carbon stretching (nu(M-CO)) vibrations in CO-containing aqueous electrochemical and gaseous environments. The effective gas-phase surface potentials extracted by matching the vibrational frequencies with the corresponding potential-dependent electrochemical spectra are substantially (ca. 1-1.5 V) lower than the work functions for such interfaces under "clean" (ultrahigh vacuum) conditions. These disparities are ascribed to the occurrence of electrochemical-like redox half-reactions in the ambient-pressure gas-phase environment, leading to surface charging, and, hence, marked alterations in the surface potential as controlled by potential-dependent redox kinetics. The possible implications of these and related findings to ambient-pressure adsorption and catalysis are discussed.