The influence of dc electric current on the electrochemical impedance and the linearity of the current-voltage relationship in ion-exchange membrane systems, has been investigated. The system under study is constituted by a membrane with negative fixed-charge and two diffusion boundary layers on both sides of the membrane, ionic transport being described by the Nernst-Planck and Poisson equations. The electrochemical impedance and a decomposition into Fourier components of the potential-time response of the system to a sine electric current superimposed on a dc electric current, have been obtained by using the network simulation method. Some of the results obtained for the electrochemical impedance are compared with the corresponding analytical expressions for ideal membranes. The great influence that the dc component of the electric current exerts on the linearity of the current-voltage relationship of an ion-exchange membrane system, is also shown. (C) 2011 Elsevier Ltd. All rights reserved.