The polymerization of a-methylstyrene (alpha MeSty) initiated by Hl/l(2) or HI in the presence of liquid sulfur dioxide has been investigated. The number-average molecular weight increased with the monomer concentration for reactions initiated by the Hl/l(2) system. l(2) also participates in the initiation process, Increasing the number-average polymer chain at higher monomer concentration. HI alone is also able to initiate the polymerization of alpha MeSty in the presence of SO2. With this initiator, transfer reaction can be minimized in systems containing low amount of SO2.