After formulation of the calculating models of mass action concentrations for metallic melts Cu Al, Ni Al and Cu Sn, it is found that in spite of their lower stability than that of compounds with congruent melting point, peritectics are popular structural units in metallic melts, neglecting their presence will make it impossible to study the thermodynamic properties of metallic melts with results which both obey the law of mass action and agree well with practice. After formulation of the calculating models of mass action concentrations for metallic melts Cu Al, Ni Al and Cu Sn, it is found that in spite of their lower stability than that of compounds with congruent melting point, peritectics are popular structural units in metallic melts, neglecting their presence will make it impossible to study the thermodynamic properties of metallic melts with results which both obey the law of mass action and agree well with practice.