Infrared reflection-absorption spectroscopic (IRAS) measurements are reported for carbon monoxide and deuterated water codosed along with potassium atoms onto Pt(111) at 95 K in ultrahigh vacuum (uhv) with the objective of elucidating the nature and roles of adsorbate and cation hydration on the electrode potential-dependent structure and bonding for the analogous Pt(111)/CO aqueous electrochemical interface. This uhv-based ternary coadsorption system was chosen in view of the availability of in-situ IRAS data for the electrode-solution interface at negative electronic charges, thereby enabling the validity of the "uhv electrochemical modeling approach" to be directly assessed. Varying the potassium dosage in the uhv system is analogous to charging the electrochemical double layer since adsorbed alkali cations are formed along with the metal electronic charge. Variations in the metal-uhv surface potential attending alterations in the interfacial composition were evaluated with a Kelvin probe : besides yielding additional insight into surface solvation, the measurements provide the required link to the in-situ electrode potential scale. Indeed, decreasing the surface potential by progressively increasing the K+ coverage at high water dosages yields potential-dependent C-O stretching (v(CO)) bands for adsorbed CO that closely mimic corresponding IRAS data for the in-situ electrochemical interface. Infrared spectra in the O-D stretching (v(OD)) region for D2O as well as the v(CO) bands are reported as a function of CO, D2O, and K+ coverage in order to explore how the combined and controlled presence of the ions and solvent acts to modify the structure and bonding of the electrostatic double layer and the chemisorbed CO. : besides yielding additional insight into surface solvation, the measurements provide the required link to the in-situ electrode potential scale. Indeed, decreasing the surface potential by progressively increasing the K+ coverage at high water dosages yields potential-dependent C-O stretching (v(CO)) bands for adsorbed CO that closely mimic corresponding IRAS data for the in-situ electrochemical interface. Infrared spectra in the O-D stretching (v(OD)) region for D2O as well as the v(CO) bands are reported as a function of CO, D2O, and K+ coverage in order to explore how the combined and controlled presence of the ions and solvent acts to modify the structure and bonding of the electrostatic double layer and the chemisorbed CO.