We have studied the radiation grafting of polyvinyl chloride at —78° and at —196°. The reaction takes place in a glassy medium obtained by swelling the polymer in its own monomer at 25°, and subsequently cooling down the irradiation temperature. Depending on the chemical nature of the grafted monomers, the reaction may proceed either by a radical mechanism (acrylonitrile, methyl methacrylate) or by a cationic mechanism (isobutylene and styrene). The results are discussed and a reaction scheme is proposed for the initiation of cationic polymerization in the presence of chlorinated solvents.