Abstract: A thermodynamic model is proposed for calculating the mass action concentration of structural units in the Al-Ti binary melt system based on the atom-molecule coexistence theory. The standard equilibrium constants of reactions for forming three associated molecules as Al₃Ti,Al Ti,and Al₁₁Ti₅ in the Al-Ti binary melt system at the temperature of 2073 to 2273 K are calculated based on the activity reported in the literature,and the expressions of the standard molar Gibbs free energy change of reactions are obtained.Meanwhile,the calculated mass action concentrations N_Al and N_Ti of Al and Ti are compared with the reported activities of both Al and Ti in the full composition range of the Al-Ti binary melt system at different temperatures from the literature. The results show that the calculated mass action concentrations N_Al and N_Ti are in agreement with the activities a_Al and a_Ti in the reported literature. The relationship between the calculated mass action concentration N_i and the calculated equilibrium amount of substance n_i of Al₃Ti and Al₁₁Ti₅ in the Al-Ti binary melt system is rod-type. However,the relationship between the calculated mass action concertation N_AlTi and the calculated equilibrium amount of substance n_AlTi in this binary melt system is spindle-type.