A solution of butyl rubber in an inert solution such as n-hexane is halogenated in a continuous process using bromine chloride vapor and high intensity mixing means for both the halogenation and subsequent neutralization steps. Adjustment of the mole ratio of bromine chloride to isoprene, where an isobutylene-isoprene butyl copolymer is used, to less than about 1.5 to 1 prevents overhalogenation and significant molecular weight loss, while the use of bromine chloride vapor results in improved process safety.The doubly halogenated product, containing both bromine and chlorine in its structure can be vulcanized in the absence of sulfur, e.g., with zinc oxide. It is particularly useful in rubber compositions where the singly halogenated product is employed, but demonstrates product performance advantages compared to both brominated butyl rubber and chlorinated butyl rubber.