Large variations in the microstructure of 1,4-polyisoprenes, from ca. 100% cis-trialkylethylene groups, as in natural rubber (NR), to ca. 40% cis- and 60% trans-trialkylethylene groups, as in an equilibrium-isomerized NR, have little influence on the overall chemistry of vulcanization of the polyisoprenes by a N-cyclohexylbenzothiazole-2-sulfenamide-accelerated sulfur system or by a dicumyl peroxide system. The peroxide crosslinks the equilibrium-isomerized NR more efficiently than it crosslinks NR; this is attributed to the sulfur dioxide, which is used to isomerize the NR, scavenging some of the nonrubber constituents in the NR, which are known to compete with the rubber hydrocarbon for reaction with free radicals from the peroxide. By comparison with NR vulcanizates, the corresponding equilibrium-isomerized NR vulcanizates have higher values of the C2 term of the Mooney-Rivlin stress–strain equation and higher χ (polymer–swelling liquid interaction parameter) values of the Flory-Huggins equation.