The synchrotron small-angle X-ray scattering (SAXS) technique has been used to study the effect of the antisolvent CO2 on the thermodynamic properties of polystyrene (PS)/cyclohexane (CH) solutions at 34.5 degreesC and at pressures up to 30.0 bar. The molecular weights (M-w) of the monodisperse PS are 3600, 22 400, 84 000, 176 000, and 341 000. The cloud point pressures of the solutions at different solution concentration are also determined. The SAXS results show that the intramolecular radius of gyration (R-g) decreases with approaching cloud point pressure due to the increasing polymer-polymer interaction. At different antisolvent CO2 pressure, there is an exponential relationship between R-g and M-w, and the exponents change from 0.5 to 0.341 with the pressure changing from 0 to 30 bar, which means that the shape of the PS chain changes from random coil to nearly globular. The intermolecular correlation length (xi) increases dramatically as the phase separation boundary is approached, and it is 7 times larger than R-g. Near the phase separation boundary, the thermodynamic fluctuations in ternary systems lead to the formation of distinct microdomains, representing interpenetrating of macromolecules, and dramatic increase of the intermolecular correlation length.