Prussian Blue films were investigated by using ac-electrogravimetry, that is, simultaneous impedance and electrogravimetry spectroscopies at different potentials between the totally reduced form, Everitt's salt, and the mixed-valence compound, Prussian Blue, in KC1 solutions. Experimental results allow clear information on the role played by the different ions that are present in the solution to be obtained. It was found that when the film was reduced potassium ions enter the film whereas these are protons that enter at potentials more anodic than the formal potential of the redox process in KC1 solutions. It was also proved that potassium ions enter the PB film without water.