ErC2O5 crystallizes in the monoclinic Cm space group. Er(1) is bonded in a 8-coordinate geometry to one O(2), one O(3), two equivalent O(1), two equivalent O(4), and two equivalent O(5) atoms. The Er(1)-O(2) bond length is 2.39 Å. The Er(1)-O(3) bond length is 2.34 Å. There is one shorter (2.43 Å) and one longer (2.45 Å) Er(1)-O(1) bond length. There is one shorter (2.38 Å) and one longer (2.43 Å) Er(1)-O(4) bond length. There is one shorter (2.35 Å) and one longer (2.40 Å) Er(1)-O(5) bond length. There are three inequivalent C sites. In the first C site, C(1) is bonded in a trigonal planar geometry to one O(1), one O(4), and one O(5) atom. The C(1)-O(1) bond length is 1.28 Å. The C(1)-O(4) bond length is 1.24 Å. The C(1)-O(5) bond length is 1.31 Å. In the second C site, C(2) is bonded in a distorted bent 120 degrees geometry to two equivalent O(2) atoms. Both C(2)-O(2) bond lengths are 1.24 Å. In the third C site, C(3) is bonded in a distorted bent 120 degrees geometry to two equivalent O(3) atoms. Both C(3)-O(3) bond lengths are 1.27 Å. There are five inequivalent O sites. In the first O site, O(1) is bonded in a distorted T-shaped geometry to two equivalent Er(1) and one C(1) atom. In the second O site, O(2) is bonded in a bent 120 degrees geometry to one Er(1) and one C(2) atom. In the third O site, O(3) is bonded in a bent 120 degrees geometry to one Er(1) and one C(3) atom. In the fourth O site, O(4) is bonded in a 3-coordinate geometry to two equivalent Er(1) and one C(1) atom. In the fifth O site, O(5) is bonded in a 3-coordinate geometry to two equivalent Er(1) and one C(1) atom. Linkers: 1 [O]C(=O)C([O])=O. Metal clusters: 2 [Er]. The MOF has largest included sphere 4.28 A, density 3.28 g/cm3, surface area 1498.72 m2/g, accessible volume 0.10 cm3/g