The density and dimensional changes of Nafion 117 H have been measured as a function of the water content. Swelling of Nafion commences at N (≡ mol ratio water to hydrogen ion) ∼ 1.9. Some anisotropy of dimensional changes was observed. Water-sorption isotherms obey Henry's Law with a nonzero intercept indicating some water retention. The diffusion coefficient of water in Nafion and the electrical conductivity of Nafion are strong functions of the water content. The latter is exploited for the development of a humidity sensor. © 1993 John Wiley & Sons, Inc.