The solid phase transition mechanism of alpha- to beta-form crystal upon specific treating with supercritical CO2 + cosolvent on original pure a and mixed (alpha+beta) form syndiotactic polystyrene (sPS) was investigated, using wide angle X-ray diffraction and differential scanning calorimetry measurements as a function of temperature, pressure, and cosolvent content. As in the supercritical CO2, sPS in supercritical CO2 + cosolvent underwent solid phase transitions from alpha- to beta-form, and higher temperature or higher pressure favored this transformation. Due to the higher dipole moment of acetone, small amounts of acetone used as cosolvent with CO2 made the transition of alpha- to beta-form occur at lower temperature and pressure than in supercritical CO2, and made the a-form crystal completely transform to beta-form in the original mixed (alpha+beta) form, whereas ethanol did not. The original beta-form crystal in the original mixed (alpha+beta) form sample acted as the nucleus of new beta-form crystal in the presence of cosolvent as it did in supercritical CO2, when compared with the original pure a-form sample. (c) 2007 Wiley Periodicals, Inc.