The crystal structure of the new synthetic compound Ba-5[B20O33(OH)(4)] . H2O was established by the methods of X-ray diffraction (a Stoe IPDS diffractometer, lambda MoKalpha, 1860 independent reflections, anisotropic refinement, R = 1.95%, localization of hydrogen atoms): a = 9.495(2) Angstrom, b = 6.713(1) Angstrom, c = 11.709(2) Angstrom, beta = 95.09(1)degrees, sp. gr. P2, Z = 1. The structure is based on double pseudohexagonal layers consisting of BO4-tetrahedra and BO3 triangles linked into three-membered rings in two mutually perpendicular directions. The double layers adjacent along the [100] direction are linked together through the Ba-polyhedra and hydrogen bonds with the participation of the OH-groups occupying the "end" vertices of two B-triangles. The interlayer space is also filled with a sheet of Ba-polyhedra. The structure of the compound is compared to the structures of topologically similar Ba and Ca berates and hydroborates. (C) 2000 MAIK "Nauka/Interperiodica". [References: 19] 19