The first three-dimensional chromium(III) dicarboxylate, MIL-53as or Cr-III(OH) (.) {O2C-C6H4-CO2}. has been obtained under hydrothermal conditions (as: as-synthesized). The free acid can be removed by calcination giving the resulting solid, MIL-53ht or Cr-III(OH).{O2C-C6H4-CO2}. At room temperature, MIL-53ht adsorbs atmospheric water immediately to give Cr-III(OH).[O2C -C6H4-CO2}.H2O or MIL-53lt (It: low-temperature form, ht: high-temperature form). Both structures, which have been determined by using X-ray powder diffraction data, are built up from chains of chromium(Ill) octahedra linked through terephthalate dianions. This creates a three-dimensional structure with an array of one-dimensional large pore channels filled with free disordered terephthalic molecules (MIL-53as) or water molecules (MIL-53lt); when the free molecules are removed, this leads to a nanoporous solid (MIL-53ht) with a Langmuir surface area over 1500 m(2)/g. The transition between the hydrated form (MIL-53lt) and the anhydrous solid (MIL-53ht) is fully reversible and followed by a very high breathing effect (more than 5 A), the pores being clipped in the presence of water molecules (MIL-53It) and reopened when the channels are empty (MIL-53ht). The thermal behavior of the two solids has been investigated using TGA and X-ray thermodiffractometry. The sorption properties of MIL-53It have also been studied using several organic solvents. Finally, magnetism measurements performed on MIL-53as and MIL-53It revealed that these two phases are antiferromagnetic with Neel temperatures T-N of 65 and 55 K, respectively. Crystal data for MIL-53as is as follows: orthorhombic space group P-nam with a = 17.340(1) A, b = 12.178(1) Angstrom, c = 6.822(1) Angstrom, and Z = 4. Crystal data for MIL-53ht is as follows: orthorhombic space group Imcm with a = 16.733(1) Angstrom, b = 13.038(1) Angstrom, c = 6.812(1) Angstrom, and Z= 4. Crystal data for MIL-53It is as follows: monoclinic space group C-2/c with a = 19.685(4) Angstrom, b = 7.849(1) Angstrom, beta = 6.782(1) Angstrom, beta = 104.90(1)degrees, and Z = 4.