Nafion (R) 211 is the membrane of reference for many fuel cell users, but the literature on its properties is sparse. Unlike previous embodiments of Nafion (R) membranes, which are obtained by melt extrusion, Nafion (R) 211 is prepared by dispersion casting. This paper reports a range of properties obtained by ex situ and in situ characterization methods. Water uptake and proton conductivity are studied as a function of temperature and relative humidity. The extent of dimensional changes of as-received membranes upon hydration is found to be independent of thermal treatment. At 25 degrees C and 97% RH the maximum hydration number is similar to 12. Absorption from liquid water and vapour are observed to be similar. Proton conductivity for Nafion (R) 211 is reported up 80 degrees C and as a function of relative humidity. At 80 degrees C, the conductivity increases from 0.0012 to 0.13 S/cm as RH is increased from 10 to 95%. The hydraulic permeability of Nafion (R) 211 is 2.4 x 10(-16) m(2) Pa(-1) s(-1) at 70 degrees C and the permeance is 8.8 x 10(-12) m Pa(-1) s(-1). Oxygen solubility and the diffusion coefficient, studied using a solid state electrochemical cell, are 1.6 x 10(-5) mol cm(-3) and 1.04 x 10(-6) cm(2) s(-1) at 30 degrees C, 100% RH, and 30 psi oxygen. Oxygen permeability is 1.71 x 10(-11) mol cm(-1) s(-1) under similar conditions. In situ measurement of hydrogen crossover current is 1.2 mA/cm(2) at 80 degrees C and 100% RH. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.