A novel equation is derived to describe the electric potential and selectivity of membranes which contain ion-exchanger and some co-exchanger. Numeric simulation of the potentials and that of concentrations of species in the membranes allows elucidation of the origin of the co-exchanger effect on potentiometric selectivity. It is shown that the elimination of the diffusion potential due to addition of co-exchanger is the main source of the beneficial effect of such additives. Another important feature of these membranes is the stabilization of the concentration of free ion-exchanging sites and their excess relative to ions. As a result, the potentiometric selectivity of membranes doped with co-exchanger becomes proportional to the association selectivity of ion-exchanging sites.