Mass impedance spectroscopy allows the modulated response of mass of a resonant electrode to be followed at different perturbation frequencies during electrochemical impedance spectroscopy experiments (ac-electrogravimetry). However, some difficulties appear if a continuous mass change takes place foreign to the mass changes due to the ac-modulated electrochemical processes. A new strategy is proposed to solve this limitation. Both, mass and potential analog signals are recorded frequency by frequency from 100 Hz up to the lowest frequencies. Then, a numerical procedure based on a multiparametric fitting of raw data against the elapsed time allows the contribution due to the modulated ac-potential perturbation to be separated from other contributions. Thus, a true impedance value is obtained. This procedure is tested and validated for electrochemical impedance spectroscopy of Prussian Blue films at potentials where no continuous electric current change has been detected. The same procedure was applied to the study of mass impedance of Prussian Yellow films where in some experimental conditions mass continuously decreases. Valuable information about the role of potassium cation and nitrate anion in the electrochemistry of this film was obtained. This procedure also opens a new way to obtain reliable kinetic interpretations in other similar cases such as electrochemical dissolution and deposition of metals as well as electrochemical polymerization or formation and dissolution of passive layers. (C) 2014 Elsevier Ltd. All rights reserved.