Abstract: Based on the atomicity and molecularity(negative deviation of activities from Raoultian behavior, minimum ΔG and ΔH of mixing at certain composition, abrupt go up of excess stability as well as maximum resistivity at compound formation composition and phase diagrams) the coexistence theory of metallic melts structure involving compound formation has been suggested. According to this theory, calculating models of mass action concentrations for different molten alloys have been deduced. The fact that calculated N_i and N_j with these models are in good agreement with measured a_i and a_j confirms that the coexistence theory of mentioned melts appropriately reflects the structural reality of these melts.