In this study, the dissociation constant of acetic acid as a weak electrolyte in an ion-exchange membrane, (K) over bar, was estimated using the Donnan equilibrium theory, which has been applied to a strong polyelectrolyte membrane. The membrane charge density and the total concentration, which represents the sum of the dissociated proton concentration and carboxyl concentration of the undissociated electrolyte in the membrane, were determined by neutral titration; the charge effectiveness was determined from membrane potential measurements; and the proton concentration in the external solution phase was determined by Donnan equilibrium theory and used for the calculation of (K) over bar. The dissociation constants, (K) over bar, in the cation exchange membrane were on the order of 10(-4) and were larger than that in water (= 1.73 x 10(-5)). The value of (K) over bar increased with an increase in the concentration in the external solution and decreased with the fixed charge density increase. It is suggested that the fixed charge group depresses the dissociation of the weak electrolyte.