Potentiometric selectivities of a liquid anion-exchange membrane containing triphenyltin chloride (TPTCl) to several inorganic anions were evaluated via measurements of the membrane potential of a bi-ionic system, also called bi-ionic membrane potential. Addition of TPTCl to the liquid anion-exchange membrane, based on the quaternary ammonium salt, gave rise to a quite different selectivity pattern from the so-called Hofmeister anion series observed for the liquid anion-exchange membrane. An additivity rule of the bi-ionic membrane potential was observed to hold for the Liquid anion-exchange membrane containing TPTCl. Thus, the following multiple chain rule was derived for selectivity coefficients; k(1,n)(pot) = K-1,2(pot) . K-2,3(pot) ... K-i,K-(i+1)(pot) ... K-n-1,n(pot), where K-i,i+1(pot) is the selectivity coefficient of the membrane for the (i+1)th ion over the ith ion.