Dynamics of water absorption from a saturated vapor and water desorption into dry air for Nafion 1100 EW ionomers have been measured for film thicknesses between 51 and 606 μm and at temperatures ranging from 30 to 90 °C. Water absorption and desorption exhibit two distinct non-Fickian characteristics:  (1) desorption is 10 times faster than absorption and (2) the normalized mass change does not collapse to a single master curve when plotted against time normalized by membrane thickness squared, t/𝓁 2, for either absorption or desorption. Water desorption data were fit well by a model in which diffusion is rapid and interfacial mass transport resistance is the rate-limiting process for water loss. Water absorption is described by a two-stage process. At early times, interfacial mass transport controls water absorption, and at longer times, water absorption is controlled by the dynamics of polymer swelling and relaxation.