The action of ultra-violet radiation on rubber has been the object of a long series of investigations. According to van Rossem, rubber is depolymerized under the action of light. Asano on the other hand thinks that ultra-violet light, is able to bring about either polymerization or depolymerization according to its wave-length. More recently Dogadkin and Pantschenkov have carried out experiments in an atmosphere of nitrogen, during the course of which they have found a strong diminution in the viscosity. From this fact they have concluded that light is able to cause depolymerization and micellar degradation. We have undertaken a study of the action of ultra-violet light on rubber in order to prove whether the double cis-linkages of rubber undergo a transposition into trans-linkages, for numerous instances are known where light causes these cis-trans-transpositions. In the case of rubber, one should obtain, therefore, either a hydrocarbon of the gutta-percha type or, if light causes a sort of cis-trans-equilibrium, a hydrocarbon with double cis-linkages distributed irregularly. In our experiments we were extremely careful to exclude oxygen, since some years ago Henri proved that ultra-violet light activates greatly the oxidation of rubber. On the other hand it is known that oxidation causes a diminution in the length of the chains which modifies considerably the physical properties, for example, the viscosity, and which may mask the effect produced by light.