Based on the phase diagrams, measured activities and the annexation principle, calculating models of mass action concentrations for In-Bi-Cu and In-Sb-Cu melts have been formulated. Calculated results not only agree well with practical values, but also obey the mass action law, showing that the deduced model can reflect the structural reality of given melts and that the annexation principle is applicable to these two ternary metallic melts. So long as there is one from three binary systems constituting the ternary melts, in which the activities of one component exhibit positive deviation relative to Raoultian behavior, heterogeous melts inevitably would form, so heterogeneous calculating model should be used to calculate their mass action concentrations. On the contrary, if all the binary melts are homogeneous and their activities exhibit negative deviation with respect to the Raoult’s law, then the ternary melts formed from them will certainly be homogeneous,hence single phase model should be applied. Based on the phase diagrams, measured activities and the annexation principle, calculating models of mass action concentrations for In-Bi-Cu and In-Sb-Cu melts have been formulated. Calculated results not only agree well with practical values, but also obey the mass action law, showing that the deduced model can reflect the structural reality of given melts and that the annexation principle is applicable to these two ternary metallic melts. So long as there is one from three binary systems constitutes the ternary melts, in which the activities of one component exhibit positive deviation relative to Raoultian behavior, heterogeous melts inevitably would form, so heterogeneous calculating model should be used to calculate their mass action concentrations. On the contrary, if all the binary melts are homogeneous and their activities exhibit negative deviation with respect to the Raoult’s law, then the ternary melts formed from them will certainly be homogeneous, hence single phase model s hould be applied.