Water transport during sorption and desorption in Nafion® 117 samples is studied by means of dynamic model and experiments as a response of relative humidity step. The model takes into account a transfer resistance at the interface between the membrane and the surrounding medium, and considers that the water content of the membrane remains uniform at all times during sorption and desorption, which was confirmed using NMR imaging. Thus, its main hypothesis is that the membrane is in equilibrium with the humid gas located at the interface whose relative humidity is different than in the bulk. The calculated time-variations in the water content of a membrane sample (in acid form) during sorption and desorption show that desorption is faster than sorption due to the variations in the slope of the water uptake isotherm. The theoretical results and the experimental data show a good agreement which allows the determination of the mass transfer coefficient. The influence of diffusion and thermal effects are also discussed.