The methanol crossover and other mass transfer phenomena have been investigated in a free-breathing direct methanol fuel cell (DMFC). The current distribution profile along the MeOH flow channel was measured and information of local concentrations of the reacting species was obtained. The DMFC with a segmented cathode was found to be very useful for a detailed analysis of the interrelated parameters, which cause the local variations of the cell cur-rent. The connections between different operating parameters were clarified in detail for two different membranes. The measurements were done for both an experimental poly(vinylidene fluoride)-graft-poly(styrene sulfonic acid) (PVDF-g-PSSA) membrane and the commercial Nafton (R) 117 membrane, which have different methanol permeabilities. The MeOH concentration and the flow rate were varied in a wide range in order to determine their optimum values. The deviations from an even current density distribution were observed to increase as a function of MeOH concentration and decrease as a function of temperature. The power production of a free-breathing DMFC was observed to be proportional to the local oxygen concentration at the cathode side and inadequate air convection together with the MeOH crossover phenomenon was observed to decrease the cell performance locally. (c) 2006 Elsevier B.V. All rights reserved.