A study has been made of the behaviour of cupric chloride in dimethyl formamide solution in systems containing a vinyl monomer polymerizable by a free radical mechanism. Although ultra-violet irradiation of such systems has been found to cause photolysis of the cupric species to cuprous with, in some cases, the formation of insoluble polymer, no measurable rates of polymerization have been detected. This observation has been explained as due to a rapid termination reaction between the cupric species and the polymerization chain-carrying radical. That the reaction between cupric chloride in dimethyl formamide solution and polymer radicals does proceed at a rate comparable to that observed in radical-radical interactions has been verified in the case of the monomers, styrene and methacrylonitrile.