In this work, we studied in detail one of the possible limiting-steps during CO oxidation at Pt and Pt/Au electrodes, namely CO adsorption and diffusion at the electrode's surface. With this aim, we used an electroless method for Pt surface modification, which entails the spontaneous deposition of gold at Pt electrodes. We also use a home-made fast-response (four electrodes) flow cell that allows an accurate control of the electrode exposition time to CO (t(ad)) and of the time interval (t(w)) before stripping of the adsorbed CO (COad). Thus two key experimental parameters of the electro-oxidation process can be independently modified and the results evaluated. Our results are consistent with a mechanism in which CO is initially adsorbed onto active sites and then moves away by surface diffusion. As a result, the subsequent oxidative stripping elimination of CO starts with COad far away from the most active sites. The experiments carried out for Pt/Au electrodes with different fraction of electrode area covered by CO (theta(CO)) demonstrate that Au acts as a barrier, hindering the surface diffusion of CO from/to the active sites, giving raise to the multiple pecks observed when tw is low. (c) 2018 Elsevier Ltd. All rights reserved.