The analysis of the various estimation methods of thermodynamic properties phase equilibrium behavior among TPTT, MPMT and RSM models is attempted based on the solution theory and thermodynamical view point. As the effect of molecular interaction among molecules in the solutions is largely significant, it is found the TPTT（two parameters two terms） equations in general have difficulties to express various multicomponent systems due to their excessive simplification.
The MPMT （multiple parameters multiple terms） equations also possess various difficulties in application of multicomponent systems due to many unknown parameters to be estimated, furthermore, estimations of the numerous parameters are excessively difficult and complex.
To correct this situation, RSM （response surface methodology） models which represent the relationship between equilibrium temperatures and compositions of vapor and liquid phase of ternary and the other multicomponent systems are optimized.
The optimized model is satisfactory in representing independent variables contributions and molecular interactions among various variables.