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The mass flux originated when two methanol–water solutions of different methanol concentration are separated by a Nafion 117 membrane in acid (H⁺) and different alkali metal forms (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) have been measured, as a function of the methanol concentration difference. From the experimental results, the methanol and water permeabilities have been estimated for the different forms of the membrane. The results show that the cationic form of the membrane strongly influences on the methanol and water permeabilities with respect to the values corresponding to its acid form. Moreover, this influence is different for water and methanol depending on the substituted cation. This strong influence of the cationic form of the membrane on the methanol and water permeabilities could be important in relation to the development of new membranes to decrease the methanol crossover in direct methanol fuel cells.