In this work we investigate the electro-oxidation of CO on Pt(111) in alkaline solution by using Fourier transform infrared spectroscopy (FTIRS), to determine the adsorption sites of the CO, the intermediate species and the final oxidation product as a function of the applied potential. Multiple CO vibration bands (on-top, bridge and 3-fold hollow site) are observed on the Pt(111) electrode, their distribution and potential dependence being strongly dependent on the surface treatment. Spectroscopic results show that the Final reaction product is carbonate and suggest that adsorbed carbonate blocks the access of CO from the (111) terrace to the active sites (i.e., step and kink sites).