Postsynthetic modification of a porous MOF, Zn4O(trans-4,4'-stilbene dicarboxylate)(3),with Br-2 results in bromination of the stilbene units. This is a heterogeneous process, with the stereocontrol originating in the rigidity of the coordinated ligand and the porous nature of the MOF, which allows "solution-like" access to reactive sites. X-ray diffraction and N-2 sorption studies suggest that partial bromination serves to increase the stability of this interpenetrated MOF structure.