Photopolymerization of a-methylstyrene in dichloromethane solution has been carried out in the presence of diphenyliodonium hexafluorophosphate A 500W high pressure mercury lamp was used as a light source of irradiation. The conversion of the monomer to polymer and the molecular weight distribution of the resulting polymer were measured by means of gravimetry and GPC, respectively. Higher rate of polymerization and higher average molecular weight of the polymer was observed at - 78˚C than those obtained at o˚C. Furthermore, bimodal molecular weight distribution of the polymer, obtained in the early stage of the reaction, revealed the simultaneous operation of the cationic species in the free ion and ion pair state.