There are two formalisms to describe the sorption equilibrium in a polymer-mixed solvent system (TPS). In the Flory-Huggins (FH) thermodynamic model, empirical ternary interaction parameters, g T and X T, are necessary to evaluate the preferential adsorption coefficient (λ) and total sorption potential ( Y). These ternary parameters have already been defined as a function of the three binary interaction parameters, g 0 i3 ( i = 1, 2) and g 12. In the Flory-Prigogine-Patterson (FPP) formalism, thermodynamic quantities depend on the molecular characteristics ( s, α and α′), and exchange interactions of all three components simultaneously being ternary per se. A comparative analysis between both formalisms in ten TPS has allowed us, for the first time, to define s, α and α′ as functions of g 0 13 and and g 0 23. A similar study extended to four TPS in which g 0 i3 values have been previously fitted to λ and A 2 (directly related to Y) experimental results, yields s, α and α′ values adjusted as well as to these experimental results. This new methodology has been applied to n-alkane-butanone-poly(dimethylsiloxane) (PDMS) systems. This was shown that, as for other systems with PDMS as polymer, other contributions different from dissimilarities in free volume should be introduced in the simplified model of FPP.