The effect of ionizing radiations on copolymers of isobutylene and styrene has been studied. Molecules consisting only of isobutylene units degrade under radiation, whereas styrene polymer cross-links. In the copolymer the styrene units offer protection against radiation to the neighbouring isobutylene units. An estimate is given for the extent of this protection. On irradiation the copolymer at first degrades to an extent which depends on the percentage of styrene, then it cross-links partially into a gel. Even for copolymers containing 80% of styrene, protection is only partial, and after high radiation doses specimens separate into a gel and a sol fraction. It is not possible to cross-link such copolymers sufficiently to produce a completely insoluble material. Chemical analysis of the irradiated copolymer shows that the separate units are distributed approximately at random throughout the material, and do not occur in large blocks rich in styrene or isobutylene.