Iodoform has been examined as a reactive transfer agent in the radical polymerizations at 60 degrees C of styrene and methyl methacrylate using azobisisobutyronitrile as initiator. The transfer constants have been determined by a procedure depending upon the determination of the initiator fragments incorporated as end-groups in the polymers. The polymerizations are retarded by iodoform; this effect is largely caused by an impurity, most probably elemental iodine, which is generated in the polymerizing systems by decomposition of the transfer agent. It is deduced that the transfer constants for iodoform with styrene and methyl methacrylate are ca. 10 and 3, respectively.