Water transport rates through polyperfluorosulfonic acid membranes at relatively low water content (close to the fuel cell failure mode) were investigated using dynamic thermogravimetry. A model was developed to describe water transport within the membrane during the dynamic thermogravimetry experiment. Using the model, the experimental data, and a non-linear regression technique, values for the effective diffusion coefficient of water in the Dow Chemical＇s experimental membrane samples of the same equivalent weight but of two different thickness, 10.16 and 17.78 mu m, were obtained for temperatures of 60 degrees C, 80 degrees C, and 90 degrees C. Results show that the effective diffusion coefficient of water in the membrane increases with water content and temperature. Values for the partition coefficient of water in these membranes at these temperatures are determined and found to be in close agreement with published values. Finally, rehydration of polyperfluorosulfonic acid membranes is found to be controlled by both the gas/solid interface and the diffusion of water into the inner structure of the membrane.