According to the Cd-Sb alloy phase diagram, resistivity, heat capacity, partial molar entropy and viscosity,the structural units of this alloy melt have been determined as Cd and Sb atoms as well as Cd₃Sb₂, Cd₄Sb₃ and CdSb compounds.Based on these structural units and the coexistence theory of metallic melt structure involing compound formation,a calculating model of mass action concentrations has been deduced.The calculated mass action concentrations N_Cd are in good agreement with measured activities a_Cd. The Gibbs standard free energy of formatioin for above three compunds have been obtained too.