The membrane potential across cation-and anion-exchange membranes was measured for LiCl methanol-water solutions. The experimental data were analyzed on the basis of the Donnan equilibrium and the Nernst-Planck equation of ion flux, considering the effect of mean activity coefficient of electrolytes in the external solution, and the theoretical predictions agreed well with the experimental data. The effective membrane charge densities and the cation-to-anion mobility ratios in membranes were determined by a non-linear regression method. The results showed that the effective membrane charge density would decrease with the increase in the weight fraction of methanol. It is postulated that the counter-ions prefer to make ion pairs with the fixed charge groups in the mixed solvent system since the dielectric constant of methanol is smaller than water. Thus the ion pairs between the counter-ions and fixed charge groups will increase with the increase in methanol concentration, and the effective membrane charge density will decrease.