Polymerization of styrene in tetrahydrofuran (THF) and cyclohexane with CO2 as antisolvent was carried out at 335 K up to 10 MPa. The results showed that, with CO2 as antisolvent, the molecular weights and molecular weight distributions of the product could be controlled by changing the pressure of the antisolvent. The polydispersity index of the polystyrene was smaller in comparison with the product polymerized in ordinary solutions. (C) 2001 Elsevier Science B.V. All rights reserved.