Surface infrared spectra in the terminal CO stretching region are reported for CO adsorbed on ordered Pt(110) in 0.1 M HClO 4 as a function of CO coverage, θ (0.05 ≲ θ ≲ 1.0) and electrode potential, E (−0.25 V ⩽ E < 0.2 V versus SCE), primarily to explor the effects of coverage-dependent surface reconstruction. Substantial differences in the coverage-dependent CO stretching frequency, ν ι CO, were observed for CO adlayers formed directly by dosing from suitably dilute solution concentrations (~ 2 × 10 −5 M), and by partial electrooxidative removal (“stripping”) from saturated irreversibly adsorbed layers. The latter procedure typically yielded only small (≲ 5 cm −1) decreases in ν ι CO and essentially constant bandwidth (FWHM ≈ 11 cm −1) from θ = 1.0 to 0.1, the presence of coadsorbed hydrogen (at −0.25 V) and with coadsorbed water (at 0.05 V). This indicates that large domains (“islands”) of close-packed CO remain on patches of (1 × 1) substrate even down to low average coverages under these conditions. For CO adlayers formed by means of the former (“dosing”) procedure at 0.05 V, however, a separate lower-frequency ν CO band (at 2030–2052 cm −1) appears for θ ≲ 0.6, supplementing the higher-frequency feature (at 2060–2072 cm −1) and supplanting it for θ ≲ 0.4. The lower-frequency ν CO band is attributed tentatively to CO bound within (1 × 2) reconstructed Pt(110) domains, having smaller local coverages (i.e. packing densities). Voltammetric electrooxidation of this low-frequency ν CO form, as deduced from simultaneous infrared and electrochemical measurements, occurs at significantly (0.1–0.15 V) lower overpotentials than for the high-frequency form. The corresponding ν ι CO−θ behavior in the presence of coadsorbed hydrogen (i.e. at −0.25 V) is substantially different, a single ν CO band exhibiting a continuous frequency decrease from 2072 (θ = 1.0) to 2005 cm −1 (θ ≈ 0.1) being obtained. Qualitatively similar ν ι CO−θ behavior was also observed in the presence of adsorbed sulfate or chloride anions. This is attributed to co-adsorption yielding a wider and more θ-dependent distribution of CO island sizes. Significant differences are noted with the corresponding infrared spectra observed for the Pt(110)/CO in UHV at 300 K, with regard to the ν CO frequency-, band intensity-, and bandwidth-θ dependencies. The absolute ν ι CO values for the electrochemical and UHV systems also differ when they are compared at the same surface potential. These differences are attributed both to the effects of the double-layer field and to co-adsorbing components on the structure of the electrochemical CO adlayer.