To reveal the possibility of supercritical (SC)-CO2-assisted devolatilization of polystyrene, the equilibrium composition data for the CO2 phase in a styrene/CO2/polystyrene ternary system is determined by a semistatic experimental technique. The parameters in the lattice-fluid equation of state of Sanchez and Lacombe are determined for the investigated system. The distribution coefficients of styrene between the polymer and the supercritical fluid phases are investigated experimentally at 318 and 328 K over the pressure range of 12-20 MPa. The binary interaction parameter between styrene and CO2 is obtained by regression of the vapor-liquid equilibrium data. The interaction parameter between CO, and polystyrene is calculated by using the sorption data from the literature, and the interaction parameter between styrene and polystyrene is optimized by using the measured data of this study. The investigation of the distribution coefficients indicates that styrene can be removed from polystyrene by SC-CO2 at near room temperature and moderately high pressures.