An electrochemical impedance spectroscopy (EIS) technique was developed to characterize a direct methanol fuel cell (DMFC) under various operating conditions. A silver/silver chloride electrode was used as an external reference electrode to probe the anode and cathode during fuel cell operation and the results were compared to the conventional anode or cathode half-cell performance measurement using a hydrogen electrode as both the counter and reference electrode. The external reference was sensitive to the anode and the cathode as current was passed in a working DMFC. The impedance spectra and DMFC polarization Curves were systematically investigated as a function of air and methanol flow rates, methanol concentration, temperature, and Current density. Water flooding in the cathode was also examined. (C) 2009 Elsevier B.V. All rights reserved.