The electro-oxidation of CO on platinum is a reaction of fundamental interest in electrocatalysis and also in technological electrochemistry, because of its relation with fuel cells and gas sensors. In this work, electrochemical impedance spectroscopy was used to study the electro-oxidation of CO on a polycrystalline platinum rotating-disk electrode in perchloric acid solution. The results were interpreted in terms of a mechanism where the formation of oxygenated species on platinum is the limiting step.