Abstract: Reaction products of sulfur with an unsaturated reactant selected from d-limonene, 5-ethylidene-2-norbornene, styrene, and dicyclopentadiene, reacted at a temperature above the melting point of sulfur and below 159.degree. C. until chemically combined, then further reacted above the melting point of sulfur until solidified. The resulting products serve as vulcanizing agents when compounded with rubber. The unvulcanized rubber compounds exhibit less blooming and better adhesion to a variety of substrates than rubber compositions compounded with soluble or insoluble sulfur. Vulcanizates containing the present compositions and a method for increasing the adhesion of rubber to a substrate during a vulcanization process are also disclosed.