Polyisoprene (IR), poly(styrene-cobutadiene) (SBR) and IR/SBR blends were vulcanized with tetramethylthiuram disulfide (TMTD)/sulfur and 2-bisbenzothiazole-2,2＇-disulfide (MBTS)/sulfur formulations and their tensile properties were determined. MBTS vulcanized IR has inferior tensile properties to TMTD vulcanizates. This is attributed in part to main chain modification in MBTS vulcanizates decreasing the ability of chains to crystallize or to align as effective load-bearing chains under stress. A similar discrepancy is not found in SBR compounds that cannot stress-crystallize. Polybutadiene, which readily crystallizes on cooling, is used to demonstrate differences in the effect of MBTS and TMTD on the ability of chains in the vulcanizates to align. These differences are confirmed by X-ray diffraction studies of stressed IR vulcanizates. The addition of zinc stearate reduces main chain modification, promotes crystallization, and improves tensile properties. Blends have inferior properties to IR, and tests involving the pulling apart of laminates and analysis of the tear surfaces are used to illustrate that failure does not occur in adhesion, but within the IF phase close to the interface. It is argued that diffusion of curatives from SBR to the faster curing IR phase, leads to the development of a layer of highly crosslinked material in IR close to the phase boundary. Failure occurs in this layer and may be attributed to a decrease in the number of effective load-bearing chains in this region or to the shorter chains in this layer becoming taut. Less diffusion of the accelerator occurs with MBTS than with TMTD, leading to a less highly crosslinked IR zone close to the interface. Consequently, higher loads are required to initiate failure. Failure in blends is likewise considered to initiate in the highly crosslinked region in the IR phase close to the phase boundary with SBR.