A theory for bi-ionic potential, especially for bi-co-ionic potential, which is a potential difference across a membrane due to the difference in co-ions, is proposed. A high water content interpolymer membrane of poly(ethyleneimine) and poly(vinyl alcohol) was prepared as a low-charged anion-exchange membrane. The potential difference Delta psi for the system of KCl/membrane/1/2MgCl(2) or 1/2CaCl(2) was measured by changing the external electrolyte concentrations c(s) and analyzed according to the theory. The bi-co-ionic potential Delta psi increased with increasing c(s) at low concentrations, and after the potential Delta psi reached a maximum value, the potential Delta psi decreased with increasing c(s) at high concentrations; that is, a bell-shaped dependence of Delta psi was observed against c(s). This is because the low mobility of co-ions with charge number +2 in the charged membranes increases with increasing c(s) and finally reaches the mobility in the foe electrolyte solutions.