Cs2CaH8(CO3)4 crystallizes in the monoclinic C2/c space group. Cs(1) is bonded in a 9-coordinate geometry to one H(4), one O(5), one O(6), two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms. The Cs(1)-H(4) bond length is 3.34 Å. The Cs(1)-O(5) bond length is 3.27 Å. The Cs(1)-O(6) bond length is 3.34 Å. There is one shorter (3.31 Å) and one longer (3.42 Å) Cs(1)-O(1) bond length. There is one shorter (3.18 Å) and one longer (3.19 Å) Cs(1)-O(2) bond length. There is one shorter (3.17 Å) and one longer (3.41 Å) Cs(1)-O(3) bond length. Ca(1) is bonded in a distorted octahedral geometry to two equivalent O(1), two equivalent O(4), and two equivalent O(6) atoms. Both Ca(1)-O(1) bond lengths are 2.36 Å. Both Ca(1)-O(4) bond lengths are 2.34 Å. Both Ca(1)-O(6) bond lengths are 2.33 Å. There are two inequivalent C sites. In the first C site, C(1) is bonded in a distorted bent 120 degrees geometry to one O(1) and one O(2) atom. The C(1)-O(1) bond length is 1.27 Å. The C(1)-O(2) bond length is 1.26 Å. In the second C site, C(2) is bonded in a distorted bent 120 degrees geometry to one O(3) and one O(4) atom. The C(2)-O(3) bond length is 1.25 Å. The C(2)-O(4) bond length is 1.28 Å. There are four inequivalent H sites. In the first H site, H(1) is bonded in a distorted single-bond geometry to one O(5) atom. The H(1)-O(5) bond length is 1.01 Å. In the second H site, H(2) is bonded in a single-bond geometry to one O(5) atom. The H(2)-O(5) bond length is 0.99 Å. In the third H site, H(3) is bonded in a distorted linear geometry to one O(5) and one O(6) atom. The H(3)-O(5) bond length is 1.61 Å. The H(3)-O(6) bond length is 1.01 Å. In the fourth H site, H(4) is bonded in a distorted single-bond geometry to one Cs(1) and one O(6) atom. The H(4)-O(6) bond length is 1.00 Å. There are six inequivalent O sites. In the first O site, O(2) is bonded in a 1-coordinate geometry to two equivalent Cs(1) and one C(1) atom. In the second O site, O(3) is bonded in a distorted single-bond geometry to two equivalent Cs(1) and one C(2) atom. In the third O site, O(4) is bonded in a 1-coordinate geometry to one Ca(1) and one C(2) atom. In the fourth O site, O(5) is bonded in a distorted water-like geometry to one Cs(1), one H(1), one H(2), and one H(3) atom. In the fifth O site, O(6) is bonded in a 2-coordinate geometry to one Cs(1), one Ca(1), one H(3), and one H(4) atom. In the sixth O site, O(1) is bonded in a 2-coordinate geometry to two equivalent Cs(1), one Ca(1), and one C(1) atom. Linkers: 4 [O]C(=O)C([O])=O. Metal clusters: 4 [Cs] ,2 [Ca]. The MOF has largest included sphere 1.61 A, density 2.61 g/cm3, surface area 2804.81 m2/g, accessible volume 0.05 cm3/g