Liquid permeation measurements of water, methanol, and 2-propanol were carried out using a commercial cation-exchange membrane Nafion-117 (perfluorinated polyethylene with pendant ether-linked side chains terminated with sulfonated groups). The experimental permeation data are treated and analyzed using the capillary model, leading to the determination of equivalent pore radius of the membrane structure. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 1412-1416, 2009