Abstract IN THE course of a study of vulcanization it became necessary to compare the ratios of carbon, hydrogen, and sulfur in hard rubber. A search of the literature revealed no analyses for carbon and hydrogen, although many analyses for sulfur in hard rubber have been recorded. Combustion analyses were therefore made on several samples of hard rubber, and the data are recorded here. The saturation of the double bonds in the rubber hydrocarbon with sulfur should give a compound, (C 5H8S)x, which contains 32.02 per cent of combined sulfur, and some of the recorded results agree with this figure. Since it is also possible to obtain products having greater amounts of combined sulfur than that called for by the theory of addition alone, some of the sulfur must consist of sulfur of substitution (7). It is theoretically possible that substitution and addition of sulfur can go on simultaneously; as the result of these two reactions, it may happen that the total sulfur in combination will equal the theoretical amount required by addition alone. In such a case there would be some free double bonds, but attempts to determine free double bonds in several samples by the Kemp-Wijs method (5) were unsuccessful.