A microporous calcium dicarboxylate material, denoted as BioMIL-2 (BioMIL stands for Bioactive Materials from Institut Lavoisier) has been obtained under solvothermal conditions. The material, composed of calcium, a non-toxic and therapeutic element, and the experimental drug glutaric acid, exhibits an unusual, reversible hydration/dehydration behaviour with a transition from a 3-dimensional porous inorganic network to a 1-dimensional inorganic sub-network dense phase upon hydration. The structures of the dehydrated and hydrated materials have been determined via single crystals and powder diffraction analysis, respectively. BioMIL-2 or CaO4C5H6 is a novel microporous material and crystallises in a rhombohedral setting (a = 20.66(1)Å, c = 8.64(1)Å, V = 3192.4(15)Å3, space group: R), within which helical chains of edge-sharing 7 coordinate calcium polyhedra are linked together in order to form a 3-dimensional network with a 1-dimensional pore system. The pores are inaccessible to N2 as they are lined with the carbon atoms of the glutaric acid moiety. Upon hydration, the calcium helicals are separated, yielding a 3-dimensional, non-porous, chained known structure. The hydrated material, BioMIL-2-hyd (CaO4OH2C5H6), crystallises in an orthorhombic setting (a = 6.78(1)Å, b = 18.42(1)Å, c = 5.87(1)Å, V = 733.04(2)Å3 space group: P212121) with the structure built up from seven-coordinate dual-capped bipentagonal calcium polyhedra arranged into chains which are connected to each other viaglutarate anions creating a 3-dimensional hybrid structure. The thermal behaviour of BioMIL-2 has been investigated using thermogravimetric analysis and thermodiffractometry and shows a complete reversible hydration/dehydration behaviour without compromise to the structure and a thermal stability up to 573 K. The controlled addition of water to CaO4OH2C5H6 leads to a ‘butter-fly’ like transformation of the framework, from the 3-dimensional inorganic network BioMIL-2 back to the 1-dimensional inorganic sub-network phase.