Polyisoprene was vulcanized with binary accelerator systems comprising mixtures of the zinc salts of tetramethylthiuram disulfide (TMTD) and 2-mercaptobenzothiazole (MBT). Samples were heated in a DSC at a programmed rate, the reaction was stopped at points along the thermal curve, and the system was analyzed. Extractable curatives and reaction intermediates were analyzed by HPLC and the crosslink density of samples measured by swelling. DSC curves for the different systems displayed similar characteristics and their similarity to the curve obtained with the zinc salt of TMTD rather than to the curve obtained with the zinc salt of MBT suggested that the reaction was dominated by the former accelerator. This conclusion was supported by HPLC analyses of extractable curatives. A reaction mechanism for the binary system is discussed.