A stability test of a direct methanol fuel cell (DMFC) was carried out by keeping at a constant current density of 150 mA cm(-2) for 435 h. After the stability test, maximum power density decreased from 68 MW cm(-2) of the fresh membrane-electrode-assembly (MEA) to 34 MW cm(-2) (50%). Quantitative analysis on the performance decay was carried out by electrochemical impedance spectroscopy (EIS). EIS measurement of the anode electrode showed that the increase in the anode reaction resistance was 0.003 Omega cm(2). From the EIS measurement results of the single cell, it was found that the increase in the total reaction resistance and IR resistance were 0.02 and 0.05 Omega cm(2), respectively. Summarizing the EIS measurement results, contribution of each component on the performance degradation was determined as follows: IR resistance (71 %) > cathode reaction resistance (24%) > anode reaction resistance (5%). Transmission electron microscopy (TEM) results showed that the average particle size of the Pt catalysts increased by 30% after the stability test, while that of the PtRu catalysts increased by 10%. 0 2007 Elsevier Ltd. All rights reserved.