The conditions of linearity of the current-voltage relationship in ion-exchange membrane systems have been investigated using the network simulation method. A network model is proposed for the Nernst-Planck and Poisson equations describing the ionic transport through an ion-exchange membrane and the two diffusion boundary layers on both sides of the membrane. With this model and the electric circuit simulation program PSPICE(TM) , a decomposition into Fourier components of the electric current-time response, which follows the application of a sinusoidal-shaped electric potential, is obtained. The results reveal a considerable influence of the membrane fixed charge concentration on the amplitude of the different harmonics. (C) 2004 Elsevier B.V. All rights reserved.