The static sorption capsule technique has now been applied to the measurement of vapor-liquid equilibria in ternary systems consisting of a polymer, high-pressure gas and cosolvent. To carry out the analysis without knowing the vapor phase composition, assumptions were made regarding the cosolvent distribution between the liquid and vapor phases. These assumptions were shown to be justified by using the groupcontribution lattice-fluid theory equation of state (GCLF-EoS). Experiments were performed on the systems PMMA-CO2- cyclopentane and PMMA-CO2-methanol. The cosolvents were chosen based on their predicted ability to enhance CO2 solubility in the polymer. The enhancement measured agreed in general with the predictions of the GCLF-EoS.