The Pt-CO system is the only one in which a mere change of the potential at which the electrode is held while the compound to be oxidized is introduced in the cell dramatically affects the activity of the electrode. So, if the Pt electrode is held at a potential in the hydrogen region while CO is bubbled in the electrolyte, electrooxidation of dissolved CO occurs at a potential of 0.6 V vs the relative hydrogen electrode, which is considerably lower than the usual one of 0.9 V at which CO is oxidized when bubbled at open circuit. We have studied the dependence of this unique phenomenon on the number of Pt monolayers (MLs) deposited on Au. The peak potential in CVs of CO-stripping decreased with increasing number of Pt MLs, showing an increasing activity of the Pt film for CO oxidation. This is in agreement with the d-band model of Norskov, which predicts that CO binds more strongly to a Pt ML on Au than to bulk Pt. Other contributing factor to the increasing activity of the Pt film with increasing number of Pt MLs is the increasing number of Pt atoms in step and kink positions, known to be active centres for the oxidation of both adsorbed and dissolved CO. This second factor is inferred from the monotonic increase of the hydrogen charge, which indicates that the second and successive MLs become increasingly rougher. As for the electrooxidation at low potentials of dissolved CO, it was completely absent with one Pt ML, and progressively emerged with increasing number of Pt MLs, a well-defined peak appearing with four MLs. The parallel evolution of the peak of dissolved CO oxidation and of the pre-peak in CO-stripping CVs confirms that dissolved CO oxidation takes place only on those Pt atoms that have become free of adsorbed CO by its oxidation in the pre-peak. (C) 2010 Elsevier Ltd. All rights reserved.