Abstract: This paper considers the extraction process to consist of two steps: (1) the creation of a cavity in the solvent of suitable size to accomodete the solute; and (2) the introduction into the cavity of a solute molecule which interacts with the solvent. Each step is a set of thermodynamic functions ΔG~°_s=G_c+G_i+RTln(RT/V) The partial molar Gibbs free energy G_c and C_i is expressed by applying the scaled particle theory in Eqs. (6) and (7). The theoretical calculation of the partition constants of inert gases by Eqs.(1), (4)—(7), as given in Table 1, are in good agreement with experimental values.