Abstract The effects of copolymer composition and microstructure on the radiation chemistry of styrene/alkane and α ‐methylstyrene/alkane copolymers have been studied. The primary radical species formed on radiolysis of the copolymers at 77 K, and identified by ESR spectroscopy, are the same as those formed during radiolysis of the homopolymers. The yields of radicals for the copolymer are as predicted assuming that the cross‐section is proportional to the electron density of each component; however, there is some evidence of radical migration to aromatic groups at 77 K. Changes in molecular structure on irradiation were detected by using 13 C NMR spectroscopy. Evidence of the consumption of terminal double bonds, and chain scission in α ‐methylstyrene/alkane copolymers was found. Measurements of viscosity supported the mechanism of cross‐linking predominating in styrene/alkane copolymers, while in α ‐methylstyrene/alkane copolymers chain scission was the major result of irradiation. Copyright © 2003 Society of Chemical Industry