The transformation of equimolar mixtures of m-bromofluorobenzene and chlorobenzene was carried out over Cu-MFI catalysts at 400 degreesC under atmospheric pressure in a fixed bed reactor. A reversible exchange of halogen atoms occurred between m-bromofluorobenzene and chlorobenzene leading to m-chlorofluorobenzene and bromobenzene. The isomerization of m-bromofluorobenzene into o- and p-bromofluorobenzene was also observed. It was found that the activity in halogen exchange increased linearly with increasing copper content up to a loading corresponding to the cationic exchange capacity of the zeolite, while the isomerization activity decreased symmetrically. It was concluded that the halogen exchange reaction between the aromatic compounds was catalyzed by the copper species located in the cationic exchange sites of the zeolite and that the isomerization reaction was catalyzed by the residual protonic acidity.