The membrane potential and permeability of NaCl were measured for six crosslinked poly(vinyl alcohol‐co‐itaconic acid) membranes having various water contents [0.25≤H≤0.83 H2O (g)/swollen membrane (g)] and interpreted by means of an equation derived from Schlögl′s theory. Both results of the membrane potential and the permeability of the membranes having H≥0.34 could be satisfactorily interpreted by the theory. The permeability of NaCl in the membrane of H=0.25 gave, however, an increasing tendency with the decrease of the upstream concentration lower than 0.04 mol/L. It was considered that the concentration dependence reverse to what was caused by the Donnan exclusion might be due to the concentration dependence of the thermodynamic partition coefficient in nonfreezing water of the membrane. By the equation thus derived the membrane potential and the permeability of NaCl in the membrane of H=0.25 were expained qualitatively. The mobility ratios of Na+ and Cl− and the effective charge density in the membranes were discussed.