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The mass flow originated when two methanol–water solutions of different methanol concentrations are separated by a membrane has been studied for two Nafion membranes. From the experimental results, the methanol and water permeabilities have been estimated. The experiments were carried out in absence and in presence of an electrolyte in the solutions. The obtained results show that, in the absence of the electrolyte, the methanol diffusion flow is higher than the osmotic water flow at all the methanol concentration gradient established, observing that the total mass flow takes place always from the concentrated toward the dilute solution. In contrast, in the presence of the electrolyte, an inversion of the total mass flow is observed at percentages of methanol higher than 40wt.%. These experimental results could be important in relation to the problem of the methanol crossover in direct methanol fuel cells.