The methanol crossover, water transfer and power characteristics of a 40 W DMFC stack are investigated under various experimental conditions including different methanol concentrations, flow rates at anode and cathode and flow directions of reactant fluids (methanol solution and air). The performance of each cell in a stack shows some non-uniformity especially at higher current densities. Under Z-type flow direction the cells exhibit more uniform behavior than when under U-type flow. The performance of a stack supplied with I M methanol solution is much better than with 0.5 M solution. However, an increased amount of methanol crossover and water transfer across the membrane have been observed. The reactants flow rates at the anode and cathode are found to affect the fluid transfer and cell performance. At a feed of I M methanol solution and ambient air pressure the maximum power output of the stack is estimated to be about 50 W (90 mW cm(-2)). The maximum power density in a stack is about 87% of that of the optimum single cell power before being assembled into a stack. (c) 2006 Elsevier B.V. All rights reserved.