A process is described for isomerizing halogenated butyl rubber from a microstructure that is predominantly exo-methylene (secondary allylic halide) to one that is predominantly endo-halomethyl (primary allylic halide). Isomerized halobutyl rubber is a halobutyl rubber that is more reactive toward a wide range of nucleophiles, thereby supporting more efficient processes for producing a variety of butyl rubber derivatives. The process includes mixing halogenated butyl rubber and a catalytic amount of metal carboxylate and optionally heating to form isomerized halogenated butyl rubber, and may be conducted in the absence or presence of solvent.