The photopolymerization of methyl methacrylate was investigated in both iodobenzene–triethylamine and iodobenzene–Sodium thiosulfate systems. The polymerization reactions proceeded according to a radical mechanism, and were initiated by the phenyl radical which was generated by photolysis of iodobenzene. Iodine radicals are expected to recombine with each other to form an iodine molecule, which would be immediately consumed by the reaction with triethylamine or sodium thiosulfate. In an iodobenzene–triethylamine system, therefore, the polymerization of methyl methacrylate would be partly initiated by the same mechanism as in the case of the photopolymerization with the iodine–triethylamine system which was discussed in our previous paper.The photopolymerization of methyl methacrylate with polyiodostyrene–triethylamine and polyiodostyrene–Sodium thiosulfate systems yielded a graft polymer, and the graft efficiency was lower in the former system than in the latter one. These facts would also suggest that the iodine–triethylamine complex was partly involved in the initiation of the photopolymerization with the polyiodostyrene–triethylamine system.