When treated with compressed CO2, syndiotactic polystyrene (sPS) undergoes a number of solid-solid transitions that do not occur on treatment with liquid solvents. For example, planar mesophase --> beta, alpha --> beta, and gamma --> beta transitions can be brought about under appropriate conditions of temperature and CO2 pressure. In addition, the transitions of glassy sPS to the planar mesomorphic and to the a form, and the gamma --> alpha transition occur at temperatures lower than when the same transitions are effected under ambient pressure. The dissolved CO2 lowers the glass transition and the cold crystallization temperatures of sPS at the rate of -0.92 and -0.58 degrees C/atm, respectively. Crystallization kinetics from the sPS-CO2 solution follow the Avrami equation, but the value of the exponent n is lower than when crystallization is conducted under ambient pressure.