Based on Onsager's approach to nonequilibrium isothermal processes, the problem of correctness of the existing experimental determination of the integral coefficient of diffusion permeability of an ion-exchange membrane has been addressed. To this end, the well-known "fine porous membrane" model, experimental data on the diffusion of sodium chloride through the cation-exchange membrane of MK-40 into a more dilute solution, and measured differences of the spontaneous electric potential on the membrane have been used. It has been shown that the true values of the integral coefficient are larger than those found experimentally according to the conventional procedure.