Water sorption and desorption in Nafion®117 membranes in acid form were studied by means of dynamic experiments as a response of relative humidity steps. The recorded mass variations with time of the membrane samples were analyzed by means of the least squares method based on a model that takes into account an interfacial mass transfer coefficient between the membrane and the bulk humid gas, diffusion being assumed negligible. In the case of large steps in relative humidity, the model predicts that the kinetics of desorption is much faster than the kinetics of sorption. The comparisons with the experimental results for large and small steps in relative humidity show a good accordance and give the value of the corresponding mass transfer coefficient.