Cu-3(SeO4)(2)(SeO3OH)(2)(H2O)(16) (1) is a novel copper selenate hydrate and the first inorganic compound to contain linear octahedral-tetrahedral heptamers. The heptamers are formed by successive polymerization of two SeO3OH tetrahedra, two SeO4 tetrahedra, and three Cu(H2O)(4)O-2 octahedra. The linkages between the octahedra and tetrahedra are in the trans-configuration, which is in agreement with the Jahn-Teller distortion of the octahedral coordination geometry of Cu2+.