Abstract: The calculation of solubilities of substances in a supercritical fluid (SCF) is one of the most important thermodynamic calcalations. This paper discusses the characteristics of extraction with SCF. The P-V-T properties of SCF can be described by an equation of state of gases and the separation mechanism of SCF extraction is fairly similar to that of liquid-liquid extraction. Besides, this paper puts forward a physical model of SCF extraction ma-chanism. Based on this model, a general formula for calculating solubilities of substances in a SCF could be derived as fellows: This formula could be simplified and modified as a semi-empirical correlation: whers, α is an enhancement factor of solubilities. For pure substances a good agreement between experimental results and calculation data with this semi-empirical correlation, was obtained with an average error less than tan percent.