The properties of metal organic frameworks (MOFs) can be tuned by postsynthetic modification (PSM): to introduce specific functionalities after their synthesis. Typically) PSM is carried out on pendant functional groups or through metal/ligand exchange, preserving the structure of the MOP. We report herein the bromination of integral alkyne units in a pair of Zr4+ and Hf4+ MOFs, which proceeds stereoselectively in a single-crystal to single-crystal manner. The chemical and mechanical changes in the MOFs are extensively characterized, including the crystal structures of the postsynthetically brominated materials, which show a mechanical contraction of up to 3.7% in volume. The combination of stability and chemical reactivity in these MOFs leads to the possibility of tuning mechanical properties by chemical transformation while also opening up new routes to internal pore functionalization.