Phenyliododichloride, C6H5IC12, can induce photopolymerization of methyl methacrylate almost instantaneously at 40°C under visible light. The initiator order and monomer order were 0.3 and close to 1.5, respectively, for [C6H5IC12] < 5.0 × lo−4mol/L while the corresponding values were 0 and 1.0, respectively, for [C6H5IC12] > 5.0 × 10−4mol/L. Kinetic and other data indicate a radical polymerization mechanism that involves complexation of a monomer molecule with C6H5IC12prior to the radical generation step. Termination appears to take place both by a bimolecular mechanism and by a degrada-tive initiator transfer mechanism at low [C6H5IC12], but it appears to take place almost exclusively by the latter mechanism when [CeH5ICl2] is high.