Quartz crystal microbalance measurements are used to investigate the water sorption in the cast Nafion films exposed to water vapor. Equilibrium and kinetic sorption data are reported for H- and Na-form Nafion films of a thickness ca. 0.02 mum. Water sorption isotherms obtained agree well with the literature data for Nafion 117. However, the water diffusion coefficients evaluated from the initial sorption and desorption rates by using the theory of diffusion in a plane sheet are ca. 7 orders of magnitude lower than those obtained from sorption measurements in ca. 200 mum thick Nafion 117 membranes. It is concluded that the kinetics of water sorption (desorption) in the partially hydrated Nafion exposed to water vapor is controlled by the first-order process of water transfer in and out of the nanoscopic hydrophilic regions (ion clusters) for membranes thinner than 10 mum. On the other hand, the linear diffusion is the rate-determining step of water transport in thick Nafion membranes and in fully hydrated thin Nafion films.