Prussian Blue films have been studied by means of ac-electrogravimetry, electrochemical impedance spectroscopy, and electrochemical quartz crystal microbalance in different aqueous salt solutions. Impedance data was interpreted in terms of a model based on a potential drop at the electrode/film interface and a potential drop at the film/solution interface. Quantitative results obtained by the fitting of impedance and ac-electrogravimetry allow to conclude that the main contribution to the potential drop at the film/solution interface is due to the dehydration-hydration process of cations on entering or leaving the film.