Surfaces are treated as separate phases with a thickness corresponding to a monolayer. The introduction of surface chemical potentials allowed the calculation of surface concentrations and surface tensions of both miscible and immiscible liquid mixtures. This model, which incorporates Girifalco and Good's equation, was tested on the Ni-Cu, Fe-Sn, Zn-Bi, Al-Na and Fe-O systems. The calculated values compared excellently with reported literature data.The change in the aluminum-cryolite and iron-slag interfacial tensions with compositional changes, is explained by sodium enrichment of the aluminum-cryolite and by FeO enrichment of the iron-slag interfaces, respectively. The developed model for the calculation of interfacial tension was successfully applied to both systems.