It is already known that natural rubber is capable of combining, under certain conditions, with various unsaturated compounds. Bacon and Farmer, for example, have fixed maleic anhydride on rubber in solution in the presence of benzoyl peroxide. This reaction has been applied likewise to acrylic acid, to acrylonitrile and to methacrylonitrile. Bacon and Farmer carried out the reaction with solutions of rubber which were refluxed for 18 hours, without, however, attempting to avoid the extraneous effect of atmospheric oxygen. The present author was, therefore, prompted to carry out the reaction protected from air in sealed tubes into which the solvent was introduced by distillation in a vacuum according to a technique based on that employed by Moureu and Dufraisse in their studies of autoxidation. In an effort to explain the mechanism of the reactions, maleic N-methylimide was chosen as the unsaturated reagent in the reaction and p-bromobenzoyl peroxide as the catalyst. These two compounds, in virtue of the nitrogen atoms and bromine atom in their respective molecules, made it possible to determine by analysis what became of them as a result of the reaction.