A new theory is formulated for the calculation of the inter-face of two immiscible electrolyte solutions. The theory is based on the self-consistent field theory fur (chain) molecules in inhomogeneous systems developed by Scheutjens and Fleer. Complexation equilibrium between ions and carrier molecules is taken into account via the multistate character of the segments. Electrostatic interactions are also included in the theory. The self-consistent equations are derived starting with the optimization of the grand canonical partition function of the system. The set of equations is solved iteratively, giving the equilibrium distributions of the components in the system. Calculations for a model system, consisting of a liquid-membrane phase doped with a carrier molecule in contact with a simple electrolyte solution, are performed to test the applicability of the new theory. These results show that the calculations presented in this paper can be used to interpret or predict important analytical features of ion-selective electrodes.