In the context of drug delivery applications, an essentially irreversible binding of 5-FU to octahedral Al atoms is obviously undesirable, as it limits the possible release of the drug under relatively mild conditions. In this regard, the results presented here support the conclusion by Datt et al., who attributed the limited release of 5-FU from their zeolite samples to the formation of complexes between 5-FU and octahedrally coordinated Al atoms. A comparison of DFTcalculated IR spectra to experimental data showed much better agreement for those models in which 5-FU is coordinated to Aloct atoms, further corroborating the assumption that adsorption at these sites was an important, if not dominant process in the experimental samples. For the Al NMR spectra, the interpretation is not so straightforward, as the adsorption of 5-FU mostly affects the lineshape of the resonance that arises from octahedral Al atoms, whereas the isotropic chemical shift hardly changes. While the DFT-based prediction of chemical shifts is straightforward, lineshapes are influenced by a multitude of factors and therefore more challenging to compute. A general issue that affects the prediction of both IR and NMR spectra is the coexistence of many different local environments in real samples (different types of octahedral Al environments, different coordination modes of the guest molecule, etc.). As the calculations can typically cover only a few representative environments, the complexity of real samples cannot be captured in full, and some deviations in calculated spectra are therefore not unexpected. Even though the calculations presented here considered only one framework type and a low Al content, it seems likely that the findings are transferable to other zeolite frameworks and Si/Al ratios. In structures with narrower pores and/or more Al atoms, it can be anticipated that Al-coordinated 5-FU molecules may be additionally stabilised, e.g., through bonds to adjacent Brønsted acid sites or through hydrogen bonds. Similarly, it seems plausible to assume that the observations made for 5-FU in the present study may also hold for other drug molecules that can bind to octahedral Al atom through carbonyl oxygen atoms (or, potentially, other functional groups with similar characteristics). When aiming to optimise the delivery of such molecules from zeolite hosts, it seems crucial to minimise the amount of octahedral Al atoms, as an irreversible binding of the guest species to these sites will reduce the amount of drug that can be released.