The field of “Metal-Organic Frameworks” (MOFs) has grown exponentially, giving rise to nanoporous materials with promising applications in gas storage, molecular separation or catalysis. Despite the use of solvothermal synthesis, some MOFs can be obtained only as polycrystalline samples and in those cases, powder diffraction is essential to characterize them. Herein, the nanoporous iron(III) terephthalate MIL-53(Fe), has been chosen to illustrate the power of powder diffraction in the field of MOFs. Its structure can be described as a one-dimensional channel system made up of trans-linked Fe octahedra, crosslinked by the bidentate dicarboxylate. Powder diffraction has been used to solve the structures of two MIL-53(Fe) samples obtained with and without fluorine. Differences between the two structural models concern the occluded water molecules which are ordered in absence of fluorine and disordered in the other case. MIL-53(Fe) shows reversible dehydration-hydration behaviour, via a metastable anhydrous phase, yielding an anhydrous phase with reduced porosity compared to its chromium analogue. Finally, sorption of organic molecules (benzene, benzoquinone, pyridine and 2,6-lutidine) in MIL-53(Fe), stud ied by using powder diffraction data in order to understand the nature of the host-guest interactions, is discussed.