Abstract: Based on the phase diagrams and the coexistence theory of metallic melts structure involving compound formation, the calculating models of mass action concentrations for Mn-P and Fe-Mn-P melts have been formulated. The calculated mass action concentrations agree well with the corresponding measured activities, this in turn shows that the deduced models can reflect the structural characteristics of the melts concerned and there isn't any saturation of phosphorus in both melts.