The structural changes of a flexible metal–organic framework (MOF) during adsorption and desorption of guest molecules were investigated by means of in situ X-ray diffraction. The flexible MOF Fe-MIL-88B was investigated in the form of surface-grown, oriented crystal films and in bulk form, and its structural changes upon ad- and desorption of water were recorded. Due to the orientation of the grown crystals on the gold substrates, structural changes in [001] direction could be observed. For the randomly oriented bulk crystals the structural changes in all crystallographic directions were observable and the changes of the lattice constants a and c and the cell volume could be determined quantitatively by indexing the complete diffraction patterns. The MOF crystals show two distinct steps of structural changes during water sorption. During adsorption of water the lattice parameter c is slightly decreased, whereas the lattice parameter a shows a prominent two-step increase resulting in an increase of the cell volume by about 40%. The conventional volumetric water sorption measurement also shows two distinct steps in the uptake of water vapor that can be correlated with the structural changes observed in X-ray diffraction.