Abstract Although according to Bernstein, exposure of rubber solutions to light in the absence of air diminishes the viscosity of the solutions, it is the conviction of the present authors that this change takes place only when there is air over the solutions. In 1920 Porritt observed that a rubber solution sealed off from contact with air gelatinized on exposure to light. Kirchhof however found this to be true only with small quantities of rubber solutions in quartz tubes exposed to ultra-violet light. This reaction was of particular interest to the present authors because of its possible relation to the vulcanization reaction, which can be regarded as an increase in molecular size, and as the formation of chains of rubber molecules with and without sulfur. Irradiation is perhaps an effect similar to vulcanization, but is brought about by polymerizations. It was also of interest to compare irradiated and gelatinized sol rubber with gel rubber. All the experiments described in the present paper were carried out with water-clear sol rubber, which was prepared in most cases from crepe.