​Incorporation of new functionalities into metal–organic frameworks (MOFs) by post-synthetic modification is a most attractive strategy for attaining the desired properties without risking destruction of the crystalline structure chosen for the aimed application. In this work the amine group in NH₂-MIL-53(Al) was used as anchoring point for the diphenylphosphinyl (Ph₂PO–) moiety. The success of the derivatization was confirmed by ³¹P MAS NMR and DRIFT spectroscopic studies. A newly synthesized reference linker bearing the same phosphinic amide functional group was used to assist in the MOF characterization. The introduction of the bulky diphenylphosphinyl group stabilizes the large-pore form of the MOF and leads to changes in the optical properties as a consequence of the electronic influence of the PO group. The approach described here is relevant for the preparation of novel phosphorus-containing MOFs.