ErC5H8O9 crystallizes in the monoclinic P2_1/c space group. Er(1) is bonded in a 8-coordinate geometry to one O(1), one O(2), one O(3), one O(4), one O(5), one O(6), one O(8), and one O(9) atom. The Er(1)-O(1) bond length is 2.34 Å. The Er(1)-O(2) bond length is 2.37 Å. The Er(1)-O(3) bond length is 2.25 Å. The Er(1)-O(4) bond length is 2.29 Å. The Er(1)-O(5) bond length is 2.43 Å. The Er(1)-O(6) bond length is 2.44 Å. The Er(1)-O(8) bond length is 2.30 Å. The Er(1)-O(9) bond length is 2.41 Å. There are five 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 1.27 Å. In the second C site, C(2) is bonded in a distorted bent 120 degrees geometry to one O(7) and one O(8) atom. The C(2)-O(7) bond length is 1.27 Å. The C(2)-O(8) bond length is 1.27 Å. In the third C site, C(3) is bonded in a distorted trigonal non-coplanar geometry to two equivalent H(3,4) and one O(5) atom. Both C(3)-H(3,4) bond lengths are 1.10 Å. The C(3)-O(5) bond length is 1.43 Å. In the fourth C site, C(4) is bonded in a distorted trigonal non-coplanar geometry to one H(5), one H(6), and one O(5) atom. The C(4)-H(5) bond length is 1.10 Å. The C(4)-H(6) bond length is 1.10 Å. The C(4)-O(5) bond length is 1.43 Å. In the fifth C site, C(5) is bonded in a distorted bent 120 degrees geometry to one O(6) and one O(9) atom. The C(5)-O(6) bond length is 1.26 Å. The C(5)-O(9) bond length is 1.27 Å. There are seven inequivalent H sites. In the first H site, H(1) is bonded in a distorted linear geometry to one O(3) and one O(6) atom. The H(1)-O(3) bond length is 1.00 Å. The H(1)-O(6) bond length is 1.65 Å. In the second H site, H(2) is bonded in a distorted linear geometry to one O(4) and one O(7) atom. The H(2)-O(4) bond length is 1.01 Å. The H(2)-O(7) bond length is 1.63 Å. In the third H site, H(3,4) is bonded in a single-bond geometry to one C(3) atom. In the fourth H site, H(5) is bonded in a single-bond geometry to one C(4) atom. In the fifth H site, H(6) is bonded in a single-bond geometry to one C(4) atom. In the sixth H site, H(7) is bonded in a distorted single-bond geometry to one O(4) and one O(7) atom. The H(7)-O(4) bond length is 1.00 Å. The H(7)-O(7) bond length is 1.64 Å. In the seventh H site, H(8) is bonded in a single-bond geometry to one O(3) atom. The H(8)-O(3) bond length is 0.99 Å. There are nine inequivalent O sites. In the first O site, O(1) is bonded in a distorted bent 150 degrees geometry to one Er(1) and one C(1) atom. In the second O site, O(2) is bonded in a distorted bent 120 degrees geometry to one Er(1) and one C(1) atom. In the third O site, O(3) is bonded in a distorted water-like geometry to one Er(1), one H(1), and one H(8) atom. In the fourth O site, O(4) is bonded in a distorted water-like geometry to one Er(1), one H(2), and one H(7) atom. In the fifth O site, O(5) is bonded in a distorted bent 120 degrees geometry to one Er(1), one C(3), and one C(4) atom. In the sixth O site, O(6) is bonded in a distorted trigonal planar geometry to one Er(1), one C(5), and one H(1) atom. In the seventh O site, O(7) is bonded in a trigonal planar geometry to one C(2), one H(2), and one H(7) atom. In the eighth O site, O(8) is bonded in a distorted bent 120 degrees geometry to one Er(1) and one C(2) atom. In the ninth O site, O(9) is bonded in a distorted single-bond geometry to one Er(1) and one C(5) atom. Linkers: 1 [O]C(=O)C([O])=O. Metal clusters: 4 [Er]. The MOF has largest included sphere 1.68 A, density 2.87 g/cm3, surface area 2242.67 m2/g, accessible volume 0.06 cm3/g