A design of experiments (DOEs) coupled with a mathematical model was used to quantify the factors affecting methanol crossover in a direct methanol fuel cell (DMFC). The design of experiments examined the effects of temperature, cathode stoichiometry, anode methanol flow rate, clamping force, anode catalyst loading, cathode catalyst loading (CCL), and membrane thickness as a function of current and it also considered the interaction between any two of these factors. The analysis showed that significant factors affecting methanol crossover were temperature, anode catalyst layer thickness, and methanol concentration. The analysis also showed how these variables influence the total methanol crossover in different ways due to the effects on diffusion of methanol through the membrane, electroosmotic drag, and reaction rate of methanol at the anode and cathode. For example, as expected analysis showed that diffusion was significantly affected by the anode and cathode interfacial concentration, by the thickness of the anode catalyst layer and membrane, and by the diffusion coefficient in the membrane. Less obvious was the decrease in methanol crossover at low cathode flow rates were due to the formation of a methanol film at the membrane/cathode catalyst layer interface. The relative proportions of diffusion and electroosmotic drag in the membrane changed significantly with the cell current of the cell. (c) 2007 Elsevier B.V. All rights reserved.