The equation of state model developed by Lacombe and Sanchez (J Phys Chem 1976, 80, 2352) is used in the form proposed later by Sanchez and Stone (Polymer Blends, Vol. 1: Formulation, 2000; Chapter 2) to correlate experimental vapor-liquid equilibrium (VLE) data for the three binaries and the ternary systems. Experimental data from the binary systems carbon dioxide-isopropyl alcohol (CO2-IPrOH), isopropyl alcohol-polystyrene (IPrOH-PS), and carbon dioxide-polystyrene (CO2-PS) are used to calculate VLE properties for the ternary system CO2-IPrOH-PS. Two-dimensional VLE-phase diagrams were calculated and used to describe from a thermodynamic point of view the pressure, volume, and temperature values that characterize a thermoplastic foam evolution process, from the extruder to the foaming die. For different initial mixture CO2 + IPrOH concentrations, pressure reduction produces liquid foaming until the vitrification curve arrests the final foam volume expansion. The dependence of the vitreous transition with the system CO2 + IPrOH concentration while foaming is represented by the Chow (Macromolecules 1980, 13, 362) equation. The calculation procedure is proposed as a design tool to reduce the amount of experimental data usually needed as a requirement previous to the design stage. (c) 2007 Wiley Periodicals, Inc.