This work aims to shed light on the wide dispersion of the values of the area of Pt/C electrodes reported when evaluated by means of the thin-film electrode approach. The effect of the Perfluorosulfonic Ionomer (PFSI) content of the electrodes and the nature of the electrolyte are discussed. The results disclose that the area of the Pt electrodes evaluated by electrochemical techniques is related to the actual PFSI content on the electrode and to the nature of the electrolyte. Using HClO4 as electrolyte, electrode area values are independent of the PFSI content. On the contrary, if experiments are recorded in H2SO4, the electrode area value increases with the increasing PFSI content, irrespectively of the Pt loading. Such effect is ascribed to the interaction of the sulfonic groups from the PFSI with the surface of the Pt nanoparticles, avoiding the strong adsorption of the bisulfate anions. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.