The effect of the membrane diffuse layer on the differential capacitance of the diffuse layer in the diffusion boundary layer of an ion-exchange membrane system, as well as on the meaning of this capacitance in the interpretation of the electrochemical impedance, have been investigated. On the one hand, the effect of the diffuse layer in the membrane phase on the electric double layer capacitance is analysed on the basis of the Poisson-Boltzmann equation in the Gouy-Chapman theory for a symmetric binary electrolyte. The results are compared with the analytical expressions obtained by using the Debye-Huckel approximation and by considering ideal membranes with an infinitely thin diffuse layer. On the other hand, the electrochemical impedance of a diffusion layer adjacent to an ideal membrane for a uni-univalent electrolyte has been numerically studied on the basis of the Nernst-Planck and Poisson equations, including a charge transfer resistance between solution and membrane described by a ChangJaffe equation, in order to establish the role played by the differential capacitance of the electric double layer. The impedance has been studied by using the network simulation method and the results obtained from the characteristic frequency of the interfacial charge transfer process are compared with the theoretical expressions for the electric double layer capacitance of an ideal system, not only under equilibrium conditions but also by considering the polarization concentration phenomenon. (C) 2015 Elsevier Ltd. All rights reserved.