This paper describes methanol flux measurements across Nafion, 1100 equivalent weight membranes under conditions of a direct methanol fuel cell but in which methanol is completely electro-oxidized on the opposite side in an inert atmosphere at sufficiently high electrode potential. Both the diffusion coefficient and the methanol concentration in the membrane were determined from the measured transient limiting current density following a potential step. Corrections for electro-osmotic drag effects are developed and found necessary even for low MeOH concentrations. The results agree well with those obtained from nuclear magnetic resonance measurements. The partition coefficient [p = [MeOH](membrane)/[MeOH](solution)] was approximately constant for the membranes in contact with methanol solutions of various concentration and from room temperature to 90 degrees C. The activation energy of methanol diffusion in a fully hydrated Nafion membrane between 30 and 130 degrees C is 4.8 kcal mol(-1), and that for protonic conduction under the same conditions is 2.3 kcal mol(-1). For a membrane dried in vacuum at above 100 degrees C, lower values of methanol permeation rate and protonic conductance were found.