AbstractA method of analysis of copolymerization systems involving multiple reaction pathways is presented. The method involves fractionation and separate analysis of the individual product populations comprising each sample. Modification of the Kelen–Tüdos linearization techniques into the form necessary to deal with high conversion copolymerizations involving multiple reaction pathways is presented. The method is applied to the ionizing radiation initiated copolymerization of styrene derivatives in dichloromethane, in which case simultaneous copolymerization via cationic and free radical mechanisms is found to occur. Reactivity ratios corresponding to each mechanism are calculated and discuss