This paper presents a generalized methodology to establish a thermodynamic model that describes a ternary mixture system, which includes a blend of two gases and a polymer melt. The model can be derived based on a phase equilibrium condition and an Equation of State (EOS). As a case study, a model for the ternary system that contains an HFC-134a/HFC-152a blend dissolved in a polystyrene (PS) melt has been developed. Using the proposed model, the solubility pressure (P-solubility) to retain a specific amount and composition of HFC-134a/HFC-152a blend in the PS melt was determined. Furthermore, the vapor composition inside a nucleated bubble during an extrusion foaming process was predicted. The derived information would aid in the control and understanding of the extrusion foaming process. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.