Abstract 1. As the percentage of sulfur in a rubber-sulfur ebonite is progressively increased, the yield temperature rises to a maximum and then falls again. There is thus an optimum proportion of sulfur for producing a heat-resistant ebonite. The cause of the reversion on exceeding this optimum has not yet been studied ; it is suggested, though without proof, that it may be due to the large amount of sulfur left uncombined. The swelling time (period required for the swelling in benzene to reach a given fraction of its maximum) likewise passes through a maximum at about the same sulfur ratio, and possibly for the same reason. 2. During prolonged vulcanization ebonite may become much harder and more heat-resistant, with little increase in combined sulfur. 3. Plastic yield temperature, hardness (as measured by Young's modulus), and absorption of petroleum hydrocarbons (petroleum ether, paraffin, transformer oil) are not determined solely by the combined sulfur content of the ebonite, but depend on how this has been attained, i.e. on the proportion of sulfur in the mix and the period of vulcanization. Increase of combined sulfur by prolonging vulcanization is more effective in producing the characteristic properties of ebonite (high yield temperature, hardness, resistance to petroleum hydrocarbons) than the same increase brought about by adding more sulfur. 4. In attempting to improve the heat-resistance of unloaded rubber-sulfur ebonites it is useless to increase the proportion of sulfur beyond the maximum now generally used, i.e. about 35 per cent of the mix, even when an accelerator is used to promote its combination with the rubber. The fact that ebonite can be made more heat-resistant without increase of combined sulfur, suggests a line along which improvement may be sought. 5. The results confirm the previous observation that well vulcanized ebonite is practically unaffected by oils of the petroleum (aliphatic) type, even at 90°– 100° C. The swelling action of such liquids, however, increases rapidly when the degree of vulcanization of the ebonite is reduced. The proportion of combined sulfur needed to confer good swelling resistance varies with the nature of the liquid, but can be deduced from the data recorded. There should be no difficulty in making an ebonite to give satisfactory service, so far as swelling resistance is concerned, in contact with oils, provided these consist of aliphatic hydrocarbons. As, however, aromatic hydrocarbons, e.g. benzene, toluene, xylene, have a strong swelling action, equally good resistance cannot be expected towards oils of appreciable aromatic content. Hydrocarbons of the cycloparaffin series (naphthenes) appear to be relatively innocuous.