Landells and Whewell have previously reported that vinyl monomers are polymerized “within viscose filaments” by hydrogen peroxide in the presence of ferrous salts and the polymers are not extractable. Such products, obtained by polymerization of acrylonitrile and styrene in viscose, have now been acetylated. Fractionation of the acetylated products has yielded graft copolymers of polyacrylonitrile and polystyrene, respectively, on cellulose triacetate. It is concluded that hydroxyl radicals initiate graft polymerization of acrylonitrile and styrene on cellulose. A similar approach, utilizing acetylation and fractionation, has been used to demonstrate that Mino and Kaizerman's method of initiation of graft polymerization with ceric salts is applicable to cellulose and results in true graft polymerization.