Different amounts of Nafion loadings (in the range of 0-2.0 mg cm(-2)) were added to a catalyst containing 0.5 mg cm(-2) of pt; these were prepared by spraying a Nafion solution on an electrode surface. The effect of Nafion loading on the activity of the catalyst and the performance of a proton exchange membrane fuel cell (PEMFC) was investigated by using electrochemical methods such as direct current polarization (using an I-V curve), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and linear scan voltammetry (LSV). The results of the I-V and EIS were compared in order to resolve the ohmic resistance (R(Omega), calculated from the T-V curve) into interfacial and internal resistances (R(if) and R(S), simulated from the EIS). The analysis of the electrochemical data revealed that the interfacial resistance (R(if)) is closely related to the reactive region of three-phase zones (interfaces among the reactants, electrolyte and catalyst), and it provides a major parameter for diagnosing the activity of the catalysts and the performance of the PEMFC. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.