Ti(O3P-(CH2)(n)-PO3) or MIL-25(n) (n = 2, 3) were prepared under hydrothermal conditions (4 days, 463 K, autogenous pressure). Their structures were determined ab initio from X-ray diffraction powder data. MIL-25(2) is triclinic (space group P-1 (no. 2)), with a = 5.033(1), b = 5.092(l), c = 6.859(l) Angstrom, alpha = 95.860(1)degrees, beta = 99.994(l)degrees, gamma = 118.217(l)degrees, and Z = 2. MIL-25(3) exhibits an orthorhombic symmetry (space group Cm2m (no. 38)), with a = 5.230(l), b = 8.451(l), c = 17.400(2) Angstrom, and Z = 4. Their three-dimensional structures are built up from TiO6 titanium(IV) octahedra linked together via diphosphonate groups. This leads to pillared structures whose inorganic sheets are closely related to those of the alpha TiP titanium phosphate structure.