Abstract In the thermochemical water-splitting iodine–sulfur process for hydrogen production, an electromembrane process (electro-electrodialysis) was applied to increase the HI molality of a HI–I2–H2O mixture. In order to investigate the temperature dependence of the membrane performance for increasing the HI concentration, a theoretical formula for the performance indexes (proton transport number, initial cell voltage, and ratio of water permeation to proton permeation in the membrane) was derived on the basis of the Nernst–Planck equation and electrophoresis theory. The obtained formula could reproduce the experimental values of the grafted membranes and Nafion 212 in the temperature range 313–373K and was validated. The temperature dependence of the performance indexes was then clarified as follows: (1) The initial cell voltage decreased with temperature upon increasing the diffusion coefficient of H+ and (2) the proton transport number decreased with temperature because the diffusion coefficient of I− increased more markedly than that of H+. For the ratio of water permeation to proton permeation in the membrane, we find the optimum regression parameters for achieving low water permeation, and controlling the ion exchange capacity might help in optimizing the membrane structure so that water permeation can be minimized.