Simultaneous normalized differential reflectance spectroscopy (Delta R/R) and second harmonic generation (SHG) has been employed to follow, independently, OH and adsorbed CO (COads) on a single Pt(111) microfacet in CO-saturated aqueous perchloric acidic solutions during voltammetric cycles, leading to the oxidation of COads and subsequent readsorption of CO on the surface. The results obtained are consistent with the disruption of the root 19 x root 19R19.1 degrees phase just prior to the oxidation of adsorbed CO.