CaC2O4H2O crystallizes in the monoclinic C2/m space group. Ca(1) is bonded in a 8-coordinate geometry to two equivalent O(1), two equivalent O(3), and four equivalent O(2) atoms. Both Ca(1)-O(1) bond lengths are 2.49 Å. There is one shorter (2.50 Å) and one longer (2.53 Å) Ca(1)-O(3) bond length. There are two shorter (2.41 Å) and two longer (2.49 Å) Ca(1)-O(2) bond lengths. There are two inequivalent C sites. In the first C site, C(1) is bonded in a distorted bent 120 degrees geometry to two equivalent O(2) atoms. Both C(1)-O(2) bond lengths are 1.26 Å. In the second C site, C(2) is bonded in a distorted bent 120 degrees geometry to two equivalent O(1) atoms. Both C(2)-O(1) bond lengths are 1.27 Å. H(1) is bonded in a single-bond geometry to one O(1) and one O(3) atom. The H(1)-O(1) bond length is 1.72 Å. The H(1)-O(3) bond length is 1.00 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded in a distorted single-bond geometry to one Ca(1), one C(2), and one H(1) atom. In the second O site, O(2) is bonded in a distorted single-bond geometry to two equivalent Ca(1) and one C(1) atom. In the third O site, O(3) is bonded in a distorted water-like geometry to two equivalent Ca(1) and two equivalent H(1) atoms. Linkers: 2 [O]C(=O)C([O])=O. Metal clusters: 2 [Ca]. RCSR code: bex. The MOF has largest included sphere 2.38 A, density 2.07 g/cm3, surface area 2248.13 m2/g, accessible volume 0.08 cm3/g