There has been a recent upsurge in interest in use of ionic liquids as reaction media for various chemical processes. Until recently, ionic liquids were considered as highly polar solvents. Our earlier investigation indicated that cationic polymerization of styrene initiated by aryl (alkyl) chlorides in ionic liquids may proceed even in the absence of coinitiator (Lewis acid). Polymerization, however, did not conform to controlled polymerization scheme. More recently, it has been claimed that ionic liquids are not as polar as it was previously assumed. Independently, high solubility of sulfur dioxide in ionic liquids was noticed. As sulfur dioxide displays a high ionizing power toward organic halides, we applied ionic liquid/sulfur dioxide mixture as a solvent in cationic polymerization of styrene initiated by aryl (alkyl) chlorides. Results show that in this reaction medium ionization of the C-Cl bond is facilitated, and the contribution of chain transfer reaction can be reduced as compared with polymerization in ionic liquid alone. Ionization of the C-Cl bond, however, is still not sufficiently fast to ensure conditions of controlled polymerization. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5251-5257, 2009