Data relating to the ZnO/ZnCl 2 -accelerated vulcanization of chlorinated poly(isoprene-coisobutylene) (CIIR or chloro-butyl) is examined. ZnCl 2 and conjugated diene butyl units on the polymer chain are both precursors to crosslinking, and a revised cationic mechanism is proposed to account for crosslinking, taking into account the involvement of conjugated diene butyl in the process. It is demonstrated that Zn 2 OCl 2 will catalyze dehydrohalogenation, and the formation of catalytic amounts of Zn 2 OCl 2 by the reaction of ZnCl 3 Ϫ with ZnO, followed by H ϩ abstraction to give Zn 2 OCl 2 and HCl, is essential in the overall crosslinking reaction sequence. The HCl is trapped by ZnO as ZnCl 2 . It is proposed that the abstraction by Zn 2 OCl 2 of HCl in a concerted reaction leads to Zn(OH)Cl and ZnCl 2 . Zn(OH)Cl remains in the polymer as an unextractable salt, while 50% of the chlorine in the rubber is extracted as ZnCl 2 when compounds reach their equilibrium crosslink density. ZnCl 2 initiates crosslinking by the abstraction of chlorine from the chain, but a crosslink will only result when a carbocation on a dechlorinated isoprenoid unit is close to a conjugated diene butyl on an adjacent chain; if not, dehydrohalogenation will result in the formation of a further conjugated diene butyl unit at that point in the chain. The maximum crosslink density achieved is only 1/4 that theoretically possible, as crosslinking restricts chain movement and limits the number of chance meetings between carbocations on the polymer and conjugated diene butyl units. Zinc stearate promotes dehydrohalogenation, ZnCl 2 being the only chloro-zinc salt formed. Reversion occurs in compounds where there is insufficient ZnO to trap all of the chlorine present in the rubber. HCl per se does not attack the polymer, but promotes reversion only in the presence of carbocations on the chain, i.e., during the crosslinking process. Trapping of HCl by ZnO prevents reversion.