The swelling capacity in these systems is primarily a function of linker length, however, this is not the only method of controlling framework flexibility; some attempts to control the breathing behaviour have focussed on linker substitution. Horcajada et al. demonstrated that when the bpdc linker in MIL-88D(Fe) is functionalised with bulky substituents such as methyl groups, the extent of breathing is reduced, as a result of the steric hindrance of the substituents reducing favourable π-π interactions between linkers as the structure closes. Similarly, for MIL-88B(Fe), where the linker is benzene-1,4dicarboxylate, di-or tetra-substitution with bulky groups imparts permanent porosity, as the MOF cannot fully close to a non-porous form. This linker functionalization strategy has also allowed control over breathing and flexibility in a series of pillared Zn MOFs. An alternative to the direct use of functionalised linkers to form MOFs is postsynthetic modification (PSM), involving a chemical change to the framework while maintaining the overall framework crystallinity. Covalent transformations of pendant groups of the organic linkers are common; this often has an effect on gas sorption properties and occasionally, flexibility of the framework. For example, Carrington et al. have recently reported the PSM of SHF-61 (SHF = Sheffield Framework), an interpenetrated In MOF which breathes in two dimensions along the crystallographic b and c axes, to yield an acetamide-modified MOF, SHF-62. The additional functionality on the pendant group of SHF-62 induces breathing in a third dimension, due to unfavourable interactions between amide substituents. In recent years, increasing attention has been paid to the PSM of integral units of MOF linkers. In 2009, Bauer et al. demonstrated the diastereoselective bromination of stilbene-4,4'-dicarboxylate (SDC) linkers in a Zn MOF, however, the reaction required high temperatures, which resulted in degradation and a loss of porosity. Marshall et al. later reported the halogenation of a range of chemically and mechanically stable Zr and Hf MOFs with ligands containing unsaturated units such as alkenes and alkynes, and successfully monitored their single-crystal to single-crystal transformations by single crystal X-ray diffraction. As well as in-depth characterisation of bulk microcrystalline samples, they reported that the brominated MOFs displayed a decrease in the average elastic modulus compared to the parent structures, proposing that the resultant change in hybridisation of the central carbon atoms of the linkers increases the degrees of freedom. Another group reported a reversible crystallineamorphous transformation of a Zr MOF containing the SDC linker upon bromination of the internal alkene unit, and also related this to increased linker motion. In these cases, PSM of integral unsaturated C-C bonds in the linkers has increased flexibility of the MOFs due to linker conformational change, but the underlying topologies do not breathe. To study the effect of integral PSM on a topologically flexible MOF, we selected MIL-88A, specifically the Sc(III) congener. Sc MOFs typically show good chemical stability, which is required to facilitate PSM, while the fumarate linker of MIL-88A(Sc) contains an accessible C=C double bond for halogenation (Figure ). In addition, we have previously shown Sc MOFs to be highly amenable to modulated selfassembly, which was expected to allow access to single crystals suitable for X-ray diffraction analysis. Herein we report the use of modulated self-assembly to synthesise MIL-88A(Sc), which we term 1, and a related, formate-bridged analogue, 2. Careful characterisation of both MOFs shows that 1 exhibits the characteristic flexibility of the MIL-88 series of MOFs, whilst 2 is rigid and permanently porous. Both MOFs can be successfully postsynthetically brominated to yield an identical final product, which further alters both flexibility and porosity.