Small concentrations of certain additives can greatly modify the effects produced by radiation of macromolecules such as polymers and biological systems. Various mechanisms of protection can be envisaged, and these lead to different kinetics in protection effects. Most published work studies the elimination of the additive, rather than the change in the macromolecule. In previous papers in this series, the dose rate effects expected for radical reactions were observed for anthracene solutions in hexane and cyclohexane. With anthracene in dimethylsiloxane polymers, however, no such dependence was observed. This problem is studied in greater detail in the present paper. The additives studied were anthracene, iodine, sulphur and benzophenone, and their protective effect on the crosslinking of dimethylsiloxane polymer was investigated at various dose rates. Contrary to generally accepted views on reaction mechanisms no dose-rate effect was observed; anthracene provided no protection against crosslinking, although it was itself destroyed. Iodine and colloidal sulphur provided a considerable measure of protection, but had no effect on gas evolution. Benzophenone also offered protection, but also reduced the gas yield. To explain these very different patterns of behaviour, it is necessary to modify some present views on the nature of the protection offered; this leads to a discussion as to the mechanism of crosslinking.