This paper investigates the analysis and design of optimal operational conditions for vapor-fed direct methanol fuel cells (DMFCs). Methanol vapor at a temperature of 35 C is carried with nitrogen gas together with water vapor at 75 C. In this experimental condition, stoichiometry of 10 is maintained for each fuel gas. The results show that the optimal operational concentration was 25-30 wt.% under methanol vapor feeding at the anode. The peak power was 14 mW cm(2) in polarization curves. To analyze major losses, the activation losses of the anode and cathode were measured by an in situ reference electrode and a working electrode. The activation loss or the anode is proportional to the water content and the high methanol concentration caused the activation loss of the cathode to increase due to methanol crossover. In the vapor-fed DMFC, the activation loss of the anode is higher than that of the cathode. Also, depending on the variation of the methanol concentration. the IR loss and Faradaic impedance is measured via impedance analysis. The methanol concentration significantly affects the IR loss and kinetics. Although the IR loss was more than the desired value at the optimal condition (25-30 wt.%), it did not significantly affect the cell's performance. The cell operated at room temperature and ambient pressure that is a typical operation environment of air-breathing fuel cells. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.