An examination has been made of the effects of 1-phenyl-1-propene, (beta-methylstyrene) (EMS), 3-phenyl-1-propene (allylbenzene) (AB), 1,2-diphenyl-1-propene (1,2-DPP) and 1,3-diphenylpropene (1,3-DPP) on the radical polymerizations of methyl methacrylate (MMA) and styrene (STY) at 60 degrees C. Transfer constants have been found by the method depending on analyses of polymers for end-groups derived from the initiator, C-14-labelled azobisisobutyronitrile (AIBN). The procedure requires information on the relative importance of disproportionation and combination in the bimolecular termination processes during the polymerizations. End-group analyses on polymers made using C-14-AIBN without additives indicate that 10% of the terminations occur by combination for MMA; the corresponding proportion for STY is 75%. AB and especially 1,3-DPP are powerful transfer agents and retarders. EMS retards the polymerizations but it appears to become incorporated inside the polymer molecules instead of forming end-groups, as it would if it functioned as a transfer agent. 1,2-DPP has almost no effect on the rates of polymerization and is only a weak transfer agent.