Brominated butyl rubber (BIIR) is a commercially important value-added elastomer that is produced by solution bromination of butyl rubber to give a microstructure that is predominantly the exo-methylene isomer, a kinetic product. It has been demonstrated that isomerization of BIIR to the thermodynamically more stable (E/Z)-endo isomers can be achieved in solution by either electrophilic (zinc stearate) or nucleophilic (tetra-n-butylammonium bromide) catalysis without significant formation of conjugated diene through competing elimination, as determined by H-1 NMR spectroscopy. Electrophilic catalysis showed little discrimination for (E/Z)-endo isomers of rearranged BIIR, whereas nucleophilic catalysis gave solely the (Z)-endo isomer. The former was interpreted as the result of thermodynamic equilibration, whereas the latter was based on a preferred reactive conformation for S(N)2' rearrangement, giving rise to a kinetically preferred product. Computational models supported the interpretation of the experimental results. Rearrangement of BIIR by electrophilic: catalysis was also demonstrated in the solid state.