Synchrotron radiation small-angle X-ray scattering (SAXS) and the viscosity technique were used to investigate the effect of dissolved CO2 in toluene on the conformation of polystyrene (PS) in the solution. The viscosity of PS solution decreases faster with increasing antisolvent CO2 pressure than that of the solvent in the absence of the polymer. The intrinsic viscosity [eta] calculated using the well-known Huggins equation decreases with antisolvent pressure. It was found that the second virial coefficient Az and the apparent mean-square radius of gyration (R-g(2))(1/2) decreases with pressure of antisolvent CO2. All these phenomena can be attributed to the shrink of PS chain in the course of adding the gas antisolvent because the interaction between the polymer and solvent becomes weaker. The values (R-g(2))(1/2) at different pressures obtained from SAXS data agree reasonably with those calculated from Flory theory using the viscosity data determined in this work. This implies that Flory theory, which has been used widely for the solutions of polymers in liquid solvents, is also applicable to the polymer solution with gas antisolvent.