Polyisoprene (IR), poly(styrene-cobutadiene) (SBR) and IR-SBR blends were vulcanized with tetramethylthiuram disulfide/sulfur in a differential scanning calorimeter (DSC) at a programmed heating rate and isothermally in a press at 130 degrees C. The reaction was stopped at various stages, and the crosslink, densities were measured. Residual curatives and extractable reaction intermediates were analyzed by high-pressure liquid chromatography (HPLC). IR crosslinked more rapidly than SBR, and the difference was attributed to the greater reactivity of the accelerator polysulfides in intitiating reaction with IR than with SBR. In blends, the greater reactivity of IR led to the earlier crosslinking of IR, the depletion of curatives in the IR phase, and the diffusion of curatives 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.