The assembly and electrochemical oxidation of well-defined CO adlayers on Pt(111) microfacets in aqueous CO-saturated 0.1 M H₂SO₄ were examined by a combination of in situ, simultaneous, time-resolved, reflectance spectroscopy (RS) and second harmonic generation (SHG), and potential step and linear scan techniques. Optical transients were collected following potential steps from a value high enough for a full monolayer of bisulfate (θ = 0.2) to adsorb on the Pt(111) facet, E_ox, to potentials E_ads, at which either the c(2 × 2)-3CO or √19 × √19R23.4°-13CO phase is expected to form once surface saturation is achieved. Similar experiments involving subsequent steps from E_ads to E_ox provided unambiguous evidence that the rates of oxidation of c(2 × 2)-3CO on such quasi-perfect Pt(111) facet at constant overpotential are much slower than those of √19 × √19R23.4°-13CO, an effect attributed to the presence of intrinsic vacant sites within the latter, more dilute phase, which are required for oxidation of adsorbed CO to ensue. Furthermore, continuous CO adsorption–oxidation cycles were found to increase the rate of oxidation of the c(2 × 2)-3CO phase. This phenomenon was tentatively ascribed to the progressive emergence of defects along the edge of the facet (and/or within the facet itself) which serve as nucleation sites for the oxidation of adsorbed CO.