A porous framework titanium(IV) N,N‘-piperazinebis(methylenephosphonate) (MIL-91(Ti)) and its aluminum analogue have been prepared under hydrothermal conditions (MIL = Material Institut Lavoisier). The structure of the aluminum analogue, AlOH(H2L)·nH2O (n ∼ 3, L = O3P−CH2−NC4H8N−CH2−PO3) was solved from a small single crystal and refined against laboratory powder X-ray diffraction data. The structure of the titanium form (TiO(H2L)·nH2O (n ∼ 4.5)) was determined using the structure of the aluminum form as a starting model and refining it against laboratory X-ray data. Their structures are built up from trans corner-sharing chains of TiO6 or AlO6 octahedra linked together in two directions via the diphosphonate groups. In each case this gives rise to a three-dimensional hybrid network with small channels along the b axis, filled with free water molecules. Thermogravimetric analysis and X-ray thermodiffractometry of the samples reveal that water is lost reversibly below 423 K and that both structures are sta...