The oxidation of carbon monoxide on a Pt(111) electrode surface is studied under transient and stationary reaction conditions in a 0.1 M HClO4 aqueous solution as a function of the applied electrode potential. The optical response is investigated by optical second harmonic generation (SHG) at 280 nm and infrared absorption spectroscopy (IRAS) starting at low potentials where the electrode is not reactive [e.g., 0.1 V vs the reversible hydrogen electrode (RHE)] towards more positive potentials where CO oxidation is initiated. For transient reaction conditions, in the absence of CO dissolved in solution, the oxidation starts at about 0.5 V vs. RHE and consists of a fast oxidation of about 10% of the adlayer and of a second reaction with slower kinetics involving the removal of the complete CO layer (overlayer stripping). Under steady-state conditions in CO-saturated solution the CO adlayer is stable up to 0.9 V vs RHE. At 0.63 V an overlayer phase transition is indicated by a 20% increase of the isotropic component of the second harmonic (SH) intensity (pp-polarization). The same potential region for this phase transition in the adlayer structure is derived from IRAS spectra after correcting for the effect of the thin layer electrolyte in IRAS measurements. The disappearance of hollow sites, the appearance of bridge sites, as well as an increased occupation of on-top sites at 0.63 V is interpreted as corresponding to the adlayer phase transition. The observations are consistent with a transition from the c(2x2) to the (root 19x root 19) adlayer structure of CO. The high sensitivity of SHG with regard to structural phase transitions of the CO adlayer is explained by distinct charge transfer contributions to the second-order surface susceptibility at different coordination sites.