Post-synthetic modification (PSM) of metal-organic frameworks (MOFs) has increased in importance in the last decade, as a pathway to access more complex surfaces in the pores and channels of porous coordination polymers. This thesis will describe new examples of tandem PSM processes leading to new functionalised MOFs that are inaccessible by direct synthesis. Chapter 1 introduces metal-organic frameworks (MOFs) and reviews the literature ranging from the basic building blocks to 3-D infinite networks. Post-synthetic modification (PSM) is introduced and a review of recent literature given. The aims of this report are also detailed at the close of the chapter. Chapter 2 contains an investigation into the conversion of primary amino to secondary amino groups in the pores of MOFs via a tandem PSM reaction. The pendent amino groups of [Zn4O(BDC-NH2)3] (IRMOF-3) and [Cr3O(OH)(OH2)2(BDC)3] (MIL-101(Cr)-NH2) were modified to produce secondary amino functionalised groups protruding into the void space. Several crystal structures are described including two obtained for the products of the PSM reaction on IRMOF-3. Nitrogen and carbon dioxide absorption was carried and high selectivity for CO2 over N2 was observed. Chapter 3 describes a new hydrothermal synthetic method of MIL-101(Cr)-NH2 and the modification, post-synthesis, to form halo- and azo dye-functionalised pore surfaces by a tandem diazotisation reaction. Quantitative yields are reported for the conversion to the halogenated frameworks inaccessible by direct synthesis with the analogous dicarboxylic acid. Gas adsorption studies demonstrated increased selectivity for CO2 over N2. Chapter 4 details the synthesis of new MOFs, with the potential for PSM, and crystallographic information is supplied for each new extended structure. The linkers, based on isophthalic acid, (1,3-benzenedicarboxylic acid) functionalised at the 5- position, were investigated with a range of metal salts and the resulting frameworks exposed to PSM reaction conditions where appropriate. By using a mixed ligand stoichiometry in the MOF synthesis reaction, 4,4’-bipy and BPDC have been incorporated into new extended frameworks. A new, simpler, synthetic route to the amino functionalised honeycomb framework [Zn4(BDC-NH2)3(NO3)2(H2O)2] (PNMOF-3) is also reported.