The dose for gelation of polymers i n aqueous solution is known to depend on polymer concentration, and several theories have been propounded. Two polymers (PVP and PEO)have been studied in detail. PVP shows an increase in gelation dose with concentration, due to a reduction in the relative importance of the indirect effect. For PEO there is no such rise, due to the absence of the indirect effect. At low polymer concentration, the gelation dose increases. It is shown that this is due to internal linking, whose magnitude depends on polymer shape, as affected by pH. This provides a method for measuring polymer configuration in solution. Radiation protection is given by various additives amo n g which certain sulphur-containing compounds are particularly effective. Each molecule can protect more than 20polymer radicals against crosslinking. To account for this high efficiency a chain reaction is assumed, whereby the protecting additive can hold H atoms, and eventually restore them to polymer radicals, producing true repair. This mechanism is important in radiobiological protection work.

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