Aluminum aminoterephthalate Al(OH)[H2N−BDC]·0.3(H2N−H2BDC (denoted MIL-53−NH2(as)) was synthesized under hydrothermal conditions. The activation of the compound can be achieved in two steps. The treatment with DMF at 150 °C leads to Al(OH)[H2N−BDC]·0.95DMF (MIL-53−NH2(DMF)). In the second step, DMF is thermally removed at 130 °C. Upon cooling in air, the hydrated form Al(OH)[H2N−BDC]·0.9H2O (MIL-53−NH2(lt)) is obtained. The dehydration leads to a porous compound that exhibits hysteresis behavior in the N2 sorption experiments. The MIL-53−NH2(lt) can be modified by postsynthetic functionalization using formic acid, and the corresponding amide Al(OH)[HC(O)N(H)−BDC]·H2O (MIL-53−NHCHO) is formed. All four phases were thoroughly characterized by X-ray powder diffraction, solid-state NMR and IR spectroscopy, and sorption measurements, as well as thermogravimetric and elemental analysis. Based on the refined lattice parameter similar breathing behavior of the framework as found in the unfunctionalized MIL-53 can be deduced. Solid-state NMR spectra unequivocally demonstrate the presence of the guest species, as well as the successful postsynthetic functionalization.