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Publisher Summary This chapter focuses on a systematic study of mixtures of the hydrocarbon–supercritical carbondioxide type to develop a single all-purpose model for solid–fluid and liquid–fluid equilibria. The chapter describes an “excess function–equation of state” model with an intermolecular interaction parameter. The interaction parameter is correlated with the parameters of interaction between the groups constituting the molecules and the fractions of sites occupied by these groups. The variation of the interaction parameter according to temperature is determined. The model is based on the relation between the excess Helmholtz energy of the mixture and the equations of state of its components. The excess Helmholtz energy is calculated in Guggenheim's quasi-reticular theory. The group contribution method is developed from experimental data concerning solid–fluid and liquid–fluid equilibria for a large number of hydrocarbons. The modelization of solid–fluid equilibria by a predictive method is more difficult because of the structural complexity of compounds of low volatility.