In this study, a 400 W-class direct methanol fuel cell (DMFC) stack is developed for large size portable applications and its operating behaviors under the various conditions are monitored. The DMFC stack comprising of 42-cells is assembled with graphite bipolar plates and membrane-electrode assemblies (MEAs) having an active area of 138 cm(2) per each. The stack is operated by varying the concentrations of methanol, stoichiometry (lambda), and the electric load. in addition, other associated factors, such as voltage and temperature distributions along the individual unit cells, pressure drops inside the stack, voltage behaviors in response to the dynamic change of the electric load and the pHs of the effluent solutions from the outlets of both electrodes, are also studied in a detailed manner. The stack produces a power of 400 W under an operating condition of feeding 0.8 M methanol and 34 l/min air at 1 atm, and uniform distributions of temperature and voltage prevail in all the 42 unit cells. A long-term operation coupled with performance restoration processes shows that a typical single cell used in this stack is able to run with a good stability for more than 500 h without any substantial degradation in the performance. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.