Polypyrrole films were investigated by using ac-electrogravimetry, that is, simultaneous impedance and electrogravimetry spectroscopies at various potentials between the reduced and oxidized forms in NaCl solutions. Information about the movement of anions, cations, and solvent during the switching of polypyrrole between oxidation states was obtained, Anions were inserted in the film, whereas cations and solvent were expelled during oxidation. Sodium ions were solvated, but the hydration number was lower at anodic potentials. The insertion/expulsion isotherms and the transfer time constants were obtained for the three species.