In-situ infrared reflection−absorption spectra (IRAS) measured for the C−O stretch (νCO) of carbon monoxide on ordered Ir(111) in aqueous 0.1 M HClO4 as a function of the dosed coverage, θCO, are compared with corresponding extant IRAS data on Ir(111) in ultrahigh vacuum (UHV) in order to elucidate the physical influences exerted by the electrochemical double layer on chemisorbate vibrational properties. Unlike most metal surfaces, CO chemisorption on Ir(111) exhibits a lone νCO band consistent with a single (atop) binding site in both electrochemical and UHV environments over the entire coverage range up to saturation and features sizable θCO-dependent changes in the νCO frequency and other spectral parameters, reflecting adsorbate−adsorbate interactions. This surface−chemisorbate system therefore provides an unusual opportunity to explore in detail solvation and related environmental factors at the same stable (unreconstructed) metal substrate in the absence of additional “chemical” effects associated with coverage- and/or solvent-induced alterations in CO binding geometry. The nature of the vibrational interactions is assessed in part by comparing the θCO-dependent spectral parameters with predictions from a numerical dipole-coupling analysis. The electrochemical νCO peak frequencies, , exhibit a marked dependence upon θCO that is comparable to that observed for the Ir(111)−UHV system. Similar saturation coverages, θCO ≈ 0.7, are also attained. Furthermore, the θCO-dependent values are approximately coincident once the differences in surface work function in these two environments are taken into account by means of a Stark-tuning analysis. However, the integrated νCO band absorbance A i displays a notably different dependence on θCO at the electrochemical and UHV-based interfaces, the former plot having a markedly less nonlinear shape than the latter. These differences can be understood in terms of solvent dielectric-screening effects, which attenuate the former A i values at low θCO. The bandwidths, Δν1/2, at the electrochemical interface are markedly (2−3-fold) larger at low and moderate θCO values compared to the UHV case. The former Δν1/2−θCO behavior is indicative of stochastic broadening associated with microscopic variations in oscillator density incurred by random chemisorption. The Lorentzian νCO band shapes (for θCO > 0.4) suggest that solvation-induced inhomogeneous line broadening is less important. The smaller Δν1/2 values and curvilinear Δν1/2−θCO behavior observed for the UHV system are consistent with local chemisorbate island formation. Nevertheless, for coverages approaching saturation (θCO ≈ 0.7) the electrochemical Δν1/2 values diminish to a value, 7 cm-1, only slightly broader than that (5 cm-1) observed for the solvent-free UHV system. The A i value attained at saturation for the electrochemical interface approaches that observed for the UHV system, the remaining dissimilarity being roughly consistent with the likely differences in the IRAS optical geometry.