A network model to represent the low-frequency electrochemical impedance of a diffusion boundary layer (DBL) in an ideal ion-exchange membrane system with simultaneous direct electric current has been proposed. This network model with elements distributed in space is obtained using the principles of the network simulation method in a system constituted by a cation-exchange membrane and two boundary layers adjacent to the membrane, from the Nernst-Planck flux equations, the electrical neutrality condition in the DBL and the Donnan equilibrium relations at the membrane vertical bar solution interface. The proposed network model is simulated by using the PSpice electrical network simulation program, not only in systems with symmetric 1:1 electrolytes but also in those with asymmetric 2:1 electrolytes. The results of the simulation for the electrochemical impedance are compared with those obtained from a simplified equivalent electric circuit with lumped parameters constituted by the series association of the ohmic resistance of the system and the well-known Warburg-type impedance. It has been found that for the highest values of the direct current, the low-frequency impedance is constituted by a straight line at relatively high frequencies and a semicircle at vanishing frequencies. The slope of the straight line is higher than 45, the radius of the semicircle is higher than that of the Warburg impedance at low frequencies, but the characteristic frequency of this arc is not a function of the direct current. (C) 2012 Elsevier Ltd. All rights reserved.