The effect of the concentration polarization on the maximum power generation in reverse electrodialysis has been studied on the basis of the Nernst-Planck-Donnan equations under electrical neutrality conditions, by means of the comparison of the optimal load and internal resistance in non-ideally selective membranes. The electric potential difference and the electric power density are numerically obtained in an ion-exchange membrane system as a function of the electric current through the load resistance. The study is done for different values of the membrane fixed-charge concentration and the thickness of the diffusion boundary layers adjacent to the membrane or the water flow velocity inside the intermembrane channels, which are related by numerically solving a 2D diffusion-convection problem, using a parabolic velocity profile, in the limiting current regime. The value of the load resistance that is needed to obtain maximum power is numerically obtained and compared to the ohmic resistance of the reverse electrodialysis stack and the internal resistances of the system obtained under maximum power transfer and short circuit conditions. (C) 2016 Elsevier B.V. All rights reserved.