The thermal stability of brominated isobutylene-isoprene rubber (BIIR) was investigated through studies of the elastomer and a model compound that accurately represented the reactive functionality within the polymer. An analysis of commercial BIIR and reaction products of brominated 2,2,4,8,8-pentamethyl-4-nonene (BPMN) by NMR demonstrated that bromination of isobutylene-isoprene rubber by 1,3-dibromo-5,5-dimethylhydantoin yielded a kinetically favored exomethylene substitution product, 3-bromo-6,6-dimethyl-2-(2,2-dimethylpropl)-1-heptane (2), as opposed to the more stable endo-isomer, (E,Z)-4-(bromomethyl)-2,2,8,8-tetramethyl-4-nonene (3). The exposure of BIIR and the brominated model compound BPMN to vulcanization temperatures led to the isomerization of 2 to 3 at a rate strongly dependent on HBr concentration. The elimination of HBr from these allylic bromides to produce exo-and endo-conjugated dienes proceeded concurrently with isomerization, and the kinetics of these processes could be rationalized on the basis of a polar reaction mechanism. The product distributions obtained from both the model system and BIIR were consistent, thereby justifying an extension of the model compound approach to an analysis of BIIR vulcanization chemistry.