When an ion-exchange membrane separates two electrolyte solutions having two different co-ions and a common counterion a bi-ionic potential (bi-co-ionic potential) appears across the membrane, if the membrane is ideally permselective for counterions and the activities of counterions on both sides of the membrane are equal to each other, the membrane potential Delta psi becomes zero. We selected KCl as a reference electrolyte because of a symmetrical electrolyte in aqueous solutions. Then, the mean activity of ions in KCI may be assumed to be equal to each single ion activity at low molalities. Single ion activity in a test electrolyte containing K+ ion or Cl- ion was estimated from the molality of KCl at Delta psi = 0 by interpolating bi-co-ionic potential to zero for KCl/membrane/LiCl, NaCl, C6H5COOK, or p-CH2CH-C6H4-SO3K systems. The values of single ion activity coefficients estimated in this work were fairly different not only from the mean activity coefficients of ions but also from the single ion activity coefficients estimated by Debye-Huckel formula.