The performance of direct methanol fuel cells containing carbon supported Pt3Co and Pt cathode catalysts with 3 nm crystallite size was investigated by AC electrochemical impedance spectroscopy under load. The electrochemical investigations were carried out at 40 degrees C and as a function of the cell voltage, oxygen vs. air feed, gas flow rate and methanol concentration. From the response of the total impedance spectra to the tested parameters, simple equivalent circuits were built and used to fit the complete DMFC impedance spectra. The electrochemical parameters (charge transfer resistance, double layer capacitance, relaxation resistance and relaxation inductance for the adsorbed species) associated with the oxygen reduction and methanol poisoning effects were determined. The AC impedance study confirmed the lower methanol tolerance of Pt3Co catalyst compared to the Pt catalyst. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.