The effect of high energy radiation on halogenated butyl rubbers was studied using electron spin resonance and nuclear magnetic resonance spectroscopies and gas chromatography. During high energy irradiation, most of the radicals were found to be generated on the halogenated isoprene units. A higher radiation yield for scission was observed in chlorinated butyl rubber than in brominated butyl rubber. This is due to the ready abstraction of hydrogen from the main chain by the chlorine radicals to give main-chain radicals which are known to contribute to scission in butyl rubbers. However, in both polymers, crosslinking predominates over scission up to a limiting dose.