AbstractTwo cases of initiation of vinyl polymerization by radical‐ions are considered. In the one case the transfer from radical‐ion to monomer is instantaneous and virtually complete. Here the relative amount of free radical propagation as compared to anionic propagation is negligibly small. In the second case a continuous feed of radical‐ion initiator is considered. Here the relative amount of free radical propagation as compared to anionic propagation may be considereable. Exact expressions are developed for this relative radical versus ionic propagation as a function of conversion. It is suggested that the results of lithium‐initiated copolymerization of methyl methacrylate and styrene may be understood in terms of a model similar to the second case of radical‐ion initiation considered in this paper.