The electrochemical impedance of ion-exchange membranes placed between two solutions with different values of the electrolyte concentration, has been investigated. The effect of the non-linear behaviour of ion-exchange membrane systems in asymmetric arrangements on the validity of impedance measurements, has also been analyzed and discussed. The system under study consists of a membrane with negative fixed-charge and two diffusion boundary layers adjacent to the membrane. The ionic transport processes are theoretically described by the Nernst-Planck and Poisson equations. The electrochemical impedance and a decomposition into Fourier components of the electric potential-time response of a cation-exchange membrane in asymmetric arrangements to a sine electric current superimposed on a direct electric current, have been obtained by using the network simulation method. The great influence that the ratio of the bathing concentrations exerts on the electrochemical impedance and the linearity of the current-voltage relationship of an ion-exchange membrane system, is shown. (C) 2011 Elsevier B.V. All rights reserved.