The kinetic mechanism of the free-radically initiated homopolymerization of acrylonitrile and methyl acrylate in dimethylsulfoxide solution has been examined. All reactions have been taken to high conversions of monomer. It has been found that the initiator efficiency is unity before the P scission of primary radicals occurs and is dependent on monomer concentration after that reaction. A kinetic mechanism that explains the changes in the initiator efficiency with conversion has been proposed, which makes use of the cage-effect concept. The proposed mechanism has made it possible to explain quantitatively the experimentally observed variation in monomer concentration with time, which has not been possible previously. It has also been found that the termination step is not significantly diffusion-controlled in the homopolymerizations.