Abstract Hitherto two products of the reaction between sulfur and rubber have been studied and used commercially, soft rubber and ebonite. Few publications have appeared concerning the products obtained by vulcanization of proportions between 5 and 30 parts of sulfur with 100 parts of rubber. Before the introduction of organic accelerators of vulcanization the coefficient of vulcanization was considered a satisfactory criterion of the quality of soft vulcanized rubber. Mixes of rubber and sulfur vulcanized to a coefficient of more than 3.5 to 4 were usually considered overvulcanized in that experience showed that the optimum properties as regards tensile strength and elongation at rupture occurred at this degree of vulcanization. Semi-ebonites differ from soft rubber and ebonite in as much as they are extremely sensitive to small changes in the time of vulcanization. Their plasticity is such that the velocity of plastic flow just prior to break is relatively great, and thus they may experience a large elongation at constant load. Their plasticity decreases with further vulcanization, in fact, with advance in vulcanization they become almost rigid at room temperature. The decrease in plastic flow is accompanied by an increase in hardness and brittleness and the ultimate stage in the rubber-sulfur reaction, ebonite, is reached.