AbstractOn irradiation of rubber sols with ultra‐violet light in the absence of oxygen, gelation takes place under certain conditions. The nature of thin reaction was studied by following its progress by means of viscosity measurements. Reaction curved determined for rubber sols in carbon tetrachloride, in the absence of accelerators, were found to be fundamentally, of the same type as those obtained in their presence. They differed, however, in showing a slight initial decrease in viscosity owing 10 degradation, probably due to traces of oxygen. The mechanism of the reaction and the possible structures of the reaction feature are discusse