In this study, the diffusion of water vapor in Nafion was measured using time-resolved Fourier-transform infrared, attenuated total reflectance (FTIR-ATR) spectroscopy. At moderate vapor activities, Fickian behavior was observed. However, at both low and high vapor activities, two different forms of non-Fickian behavior were observed. Models have been developed for both non-Fickian regimes, where a reaction between water and immobile sulfonic acid sites within the polymer has been considered at low activities and polymer relaxation has been considered at high activities. Both diffusion-reaction and diffusion-relaxation models regress well to the experimental data. In addition to modeling non-Fickian behavior at low and high vapor activities in order to extract true diffusion coefficients, the determination of accurate diffusion coefficients over the entire activity range will be explained in terms of an appropriate assessment of the vapor-polymer boundary conditions. More specifically, water sorption dynamics were a strong function of the vapor flow rate. Inaccurately accounting for this boundary condition will result in diffusion coefficients an order of magnitude lower than the true values.