Polyisoprene (IR), poly(styrene-cobutadiene) (SBR), and IR-SBR blends were vulcanized with 2-bisbenzothiazole-2,2＇-disulfide (MBTS) and sulfur in a differential scanning calorimeter (DSC) at a programmed heating rate and isothermally in a press at 150 degrees C. The reaction was stopped at various stages, crosslink densities were measured, and residual curatives and extractable reaction intermediates were analyzed by high-pressure liquid chromatography (HPLC). The reactivities of IR and SBR towards MBTS was found to be more similar than that of the rubbers towards tetramethylthiuram disulfide. In blends, the slightly greater reactivity of IR led to its earlier crosslinking, as shown by thermogravimetric analysis (TGA) of the insoluble material present at various cure times. The depletion of curatives in the IR phase led to their diffusing from SBR to IR; consequently, a zone of highly crosslinked material developed in IR close to the interface. The freezing point of a solvent, imbibed into a gel, is decreased as crosslinking proceeds; and dissimilarities in the crosslink densities of the phases in blends were demonstrated by comparing the crosslink density, calculated from swelling experiments, with the depression of the freezing point of the imbibed solvent.