The swelling and diffusion behaviour of a Nafion(R) 117 ion-exchange membrane was investigated in mixed water-methanol solutions. The membrane porosity was found as a function of the methanol content in solution. The water and methanol uptake was determined by Raman spectrometry. The permeation experiments were realized in conditions where a membrane initially separated a water-methanol electrolyte solution from a solution prepared with pure water, both solutions containing either 0.1 M HCl or 0.5 M HCl. Different rotation speeds of stirrer rods were applied in the range of 70-700 rpm. A mathematical model for water-methanol permeation through a membrane taking into account the transfer through adjacent diffusion boundary layers (DBLs) and the dependence of the methanol diffusion coefficient on the methanol concentration was developed. The DBL thickness, delta, and the methanol permeability, P*, were determined by numerical fitting of experimental data. It was found that the rotating stirrers used in the cell were almost as efficient as rotating electrodes or membranes: we have found that delta approximate to 1.2 delta(Levich), where delta(Levich) was calculated by the well-known Levich equation for rotating disc electrodes. However, the contribution of diffusion resistance of DBL into the overall resistance of the three-layer system was not negligible, even at high rotation speeds, due to high methanol permeation through Nafion(R) 117. When taking into account the DBLs, the "true" methanol permeability of Nafion(R) 117 in the presence of 0.5 M HCl in the upstream and downstream compartments (at 25 degrees C) was equal to 0.44 x 10(-5) cm(2) s(-1). If the contribution of DBLs was not considered, the apparent methanol permeability at rotation speed of 70 rpm (delta approximate to 76 mu m) was equal to 0.35 x 10(-5) cm(2) s(-1). Even at 650 rpm (delta approximate to 25 mu m), the apparent methanol permeability, (0.41 x 10(-5) cm(2) s(-1)) was still lower than its "true" value. An approximate equation relating the apparent (efficient) non-electrolyte permeability with the "true" membrane permeability and the diffusion layer thickness was deduced following the approach proposed by Helfferich. (C) 2011 Elsevier B.V. All rights reserved.