It is generally believed that CO2 and H2O acting as a supercritical co-blowing agent in the process of polystyrene (PS) foaming can offer lower foam density and uniform cell morphology for better thermal insulation. However, the solubilities of the co-blowing agent in PS have not been measured, since the dissolution equilibria are very difficult to establish in experiments. In this work, the dissolution equilibria and interfacial properties of the CO2-H2O-PS ternary system have been quantitatively predicted via a density functional theory. It is found that, in the presence of water, CO2 dissolution and PS swelling have been enhanced. For instance, after the introduction of 0.05 mass fraction of water into PS melt, which can be realized at higher pressure (over 16 MPa), CO2 solubility can be increased by 50%, whereas the corresponding interfacial tension of gas-melt can be reduced by 40%. (C) 2017 Published by Elsevier B.V.