The membrane potential of a weakly amphoteric polymer membrane was measured by varying the dissociation state of the dissociative groups within the membrane through control of the pH of the outer aqueous solution, and the effective charge density of the membrane was evaluated. On the basis of a modification of the Teorell-Meyer-Sievers equation, we concluded that the difference between the charge densities of the positively and negatively charged dissociative groups seemed to play an important role in the permeation behaviour through the membrane. From the results of a study of the potentiometric behaviour of aqueous polyamphoteric solutions, it was confirmed that the ratio of the effective charge density to the net value could be estimated to be the same as that found previously.