Abstract: The mixing rule recently developed by using the binary interaction function L_ij(T, x) depending on the thermodynamic state of a given system, coupled with FRKSequation of state, had been successfully used for calculating the excess properties of wide variety of complex systems.In this work, we further indicated that this mixing rule satisfies the invariance condition and tested the capability of this approach for correlating and predicting vapor-liquid equilibria of highly non-ideal systems at high pressure.Fifteen ternary mixtures and their constituent binaries were selected to do the test.The systems selected cover the range from almost ideal to highly non-ideal mixtures.The correlation results of the binary VLEshow that for highly non-ideal systems, instead of the single optimum L_ijvalue, L_ijfunction has to be used for correlating their VLEquantitatively.The ternary VLEwere predicted by using parameters of constituent binaries only.The results show that, in the case of simple systems, both single optimum L_ijvalue and L_ijfunction methods predict the VLEwith nearly the same accuracy; nevertheless, the L_ijfunction method shows a slight edge over the single optimum L_ijvalue method.For the complex systems containing hydrogen, polar and associated components, while the single L_ijmethod fails to represent their VLE, the L_ijfunction approach can predict the VLEfor all of these complex systems accurately.The predicted results are in good agreement with the experimental data。

Key words: Vapor-liquid equilibria, Multi-component system, Interaction function