Mechanism of solid phase transformation of alpha to beta form crystal of syndiotactic polystyrene (sPS) was investigated in supercritical CO2. The phase transformation occurred in the original pure alpha and mixed (alpha + beta) form sPS in supercritical CO2 was traced as a function of temperature and pressure by means of wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC). At appropriate temperature and pressure, sPS underwent solid phase transitions from alpha to beta form. Higher temperature or higher pressure favored this transformation. Compared to the original pure alpha form sample, the original beta form crystal in the mixed (alpha + beta) form sample acted as the nucleus of beta form crystal, so that reduced the transition temperature and pressure. (c) 2005 Elsevier Ltd. All rights reserved.