Platinum single crystal electrodes, Pt(h k l), represent ideal materials where studying surface sensitive reactions such as oxygen reduction reaction (ORR). Moreover, there is a great interest in testing carbon supported electrocatalyts mixed with Nafion (R) ionomer in order to directly evaluate catalysts under practical fuel cell conditions. Thus, we provide a first imaging attempt by scanning electrochemical microscopy (SECM) to locally evaluate the electrocatalytic activity during ORR on a Pt(1 1 1) single crystal electrode decorated with spots of commercial carbon supported platinum nanoparticles entrapped in Nafion (R). Both electrocatalysts present the same chemical composition and then, total surface area, particle size and crystallographic orientation at the electrode surface are the effects studied. Our SECM images prove that the peroxide pathway can also be considered a relevant reaction route on platinum electrodes. We agree with some recent reports pointing the Nafion (R) content and the three-dimensional surface electrode area as key factors to control for achieving a proper evaluation of the apparent number of electrons exchanged during ORR. (C) 2011 Elsevier Ltd. All rights reserved.