Membrane potentials of polysulfonated cation-exchange (SA(3)S and SA(3)T) membranes for KCl, NaCl, LiCl, CaCl2 and FeCl3 solutions using Ag/AgCl electrodes were measured. The measurements were carried out for, KCl vertical bar SA(3)S or SA(3)T vertical bar MCl, (M:K, Na, Li, Cu and Fe) systems. The concentration of salt solution (KCl) was kept constant in one side of the cell, C-1, at 1x10(-2) M and was changed in the other side of the cell, C-2, from 1x10(-4) to 2 M. The alteration graphs were drawn in order to find the potential values of membranes for ln (C-1/C-2) ratio changes. Using the values, the effective fixed charge density in membrane (C-x), the average value of transport numbers (t(+/-)), and partition coefficients (K-+/-) in a polysulfonated cation-exchange membrane for different valence ions were determined. Using the Donnan equation and the Teorrel-Meyer-Sievers theory, membrane diffusion potentials and Donnan potentials were calculated both theoretically and experimentally.