Single crystals of a new barium-manganese oxide were grown under hydrothermal conditions at (600^{\circ} \mathrm{C}) and (0.2 \mathrm{GPa}). The chemical composition determined by (\mathrm{X})-ray analysis is (\mathrm{BaMn}{3} \mathrm{O}{5}(\mathrm{OH})). The compound crystallizes in the orthorhombic system (space group (1 \mathrm{mmm}) ) with a basic cell of (a=19.355(3) \AA, b=4.531(1)) (\AA, c=2.791(1) \AA), and (Z=2). The structure exposes a supercell with twice the volume of the basic cell and monoclinic symmetry (\left(a=19.554(3) \AA, b=4.531(1) \AA, c=5.581(1) \AA, \beta=98.20(1)^{\circ}\right.), (Z=4), space group (C 2 / m) ). The crystal structure was determined by the Patterson method and refined to (R=0.038) based on 1184 independent reflections. The structure consists of ribbons composed of four parallel chains of edge-sharing (\mathrm{MnO}{6}) octahedra running along ([001]) and parallel to the ((010)) plane. The two outer chains of the ribbon are complete. One out of two (\mathrm{Mn}) ions is missing in the two inner chains. In these octabedra the cornersharing oxygens are replaced by (\mathrm{Ba}). The (\mathrm{MnO}{6}) octahedra are highly distorted and indicate the trivalent state of Mn. Valence balance at the site and large temperature factors are clear evidence of a replacement of one of seven independent oxygens by OH. 1994 Academic Press, Inc.