The effects of dosing various solvents along with potassium upon the infrared reflection-absorption spectra (IRAS) for saturated chemisorbed CO adlayers on Pt(111) at 90 K in ultrahigh vacuum (UHV) have been examined with the objective of assessing the roles of interfacial solvation upon the vibrational Stark effect (i.e., the adsorbate frequency-electrode potential dependence) for this archetypical Pt/CO electrochemical system. The solvents overlayers chosen-water, methanol, acetonitrile, acetone, and ammonia-used also in our earlier "UHV electrochemical modeling" studies, span a range of polarity and other solvating properties. Potassium dosage, with coverages theta(K) less than or equal to 0.08, yields cations with the electron transferred Co the metal surface and is therefore analogous to electrochemical double-layer charging. Kelvin-probe measurements of the work-function changes, Delta Phi, attending the addition of cations and solvent were undertaken so to evaluate the desired relationship between shifts in C-O vibrational frequencies, Delta nu(CO), and the overall surface potential. Complete solvation of the CO adlayer, usually requiring 1-2 solvent monolayers on top of the chemisorbate, induced substantial and solvent-sensitive downshifts of the atop nu(CO) frequency and the work function (up to 50 cm(-1) and 1.5 eV, respectively). Addition of solvent to saturated CO adlayers with predosed K+ yielded profound changes in the nu(CO) spectra. First, low solvent dosages, corresponding to solvent/cation stoichiometries greater than or equal to 3, essentially eliminated the markedly red-shifted nu(CO) band component due to short-range K+-CO interactions, apparently as a result of primary cation solvation. Additional solvent dosing red-shifted further the atop and bridging nu(CO) bands that are characteristic of longer-range cation-CO interactions, finally yielding the simple linear Delta nu(CO)-Delta Phi, dependence familiar for in-situ electrochemical interfaces. In contrast to tile addition of solvent to cation-free interfaces, the nu(CO) frequency shifts induced by progressive cation addition in the presence of large solvent dosages are relatively insensitive to the nature of the solvent. Furthermore, the Delta nu(CO)-Delta Phi dependences are essentially independent of the solvent for dosages, theta(s)* similar to 2 monolayers, sufficient to complete the nu(CO) spectral changes. This last finding indicates that much of the potential drop fails across the CO adlayer under these conditions, the solvent acting as a dielectric in screening the cation charge. Such simple solvent-independent Delta nu(CO)-Delta Phi behavior, which contrasts the complex spectral patterns seen upon alkalimetal addition in the absence of solvent, is in good agreement with the Delta nu(CO)-electrode potential dependences observed by in-situ IRAS at the corresponding electrochemical interfaces. The likely multifaceted robs of the solvating medium in influencing the nature as well as magnitude of the electrochemical Stark effect are discussed in light of these findings.