In different media in the range of approximately 160-240degreesC, the gamma form of syndiotactic polystyrene was transformed into different crystal forms having different stabilities. Compared with the transition to less stable forms in ambient air and gaseous CO2, the gamma form transformed into beta' form in supercritical CO2 at 240degreesC and 12 MPa, and into the more ordered beta'' form in supercritical CO2 and acetone at the same temperature and pressure.