Compounds of polyisoprene with sulfur and bis(2-mercaptobenzothiazole)zinc(II) (Zn(mbt)(2)) or ZnO and 2-mercaptobenzothiazole (MET) were vulcanized by heating in a differential scanning calorimeter. The reaction was stopped at points along the thermogram and the product analyzed. ZnO and MBT readily react, the reaction going to completion during compounding. The presence of Zn(mbt)(2) delays the onset of crosslinking compared to compounds without zinc. It is suggested that the induction period prior to crosslinking is occasioned by the inactivity of Zn(mbt)(2), which must breakdown to MBT before it can participate in the vulcanization process. Such decomposition results from attack by anions generated when polysulfidic crosslinks, formed in the unaccelerated sulfur that occurs in the early stages of crosslinking, undergo scission. The effect of MET, not bound to zinc, on the mechanism of the reaction is discussed.