AbstractThe excess enthalpy for a quasiternary system of type MkN(1‐k)XlY(1‐l)is derived by use of a model, which considers the interaction of clusters composed of four particles. The distribution of the lattice constituents is assumed to be random. Based on this model the excess enthalpy can be described by a cubic function of the mole fractionskandlwith the standard reaction enthalpy and 8 interaction parameters as constants. It is shown, that the ternary interaction parameters can be calculated from the interaction parameters of the subregular quasibinary subsystems, whereas the reaction enthalpy for the double conversion between the binary components can be derived from their standard enthalpies. Thus the phase diagrams of quasiternary systems with two mixed sublattices can be derived from the quasibinary data without introducing any special ternary interaction parameter. ‐ The influence of the interaction parameters on the shape of the phase diagram and especially the influence of the standard reaction enthalpy on the tie line field is demonstrated by some calculated phase dia