The electrochemical impedance and the linearity of the current-voltage relationship in inhomogeneous ion-exchange membrane systems, have been investigated. The system under study is constituted by a membrane with negative fixed charge varying linearly with position inside the membrane 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 current-tinge response of the system to a sine electric potential. have been obtained by using the network simulation method. The influence of the degree of inhomogeneity of a membrane on the electrochemical impedance and the linearity of the current-voltage relationship of ion-exchange membrane systems, is analyzed and discussed. (C) 2009 Elsevier Ltd All rights reserved