Sr ₉Os ₅O ₂₃, a new strontium osmate, has been synthesized as a coarse-crystalline material from SrO ₂and Os, in a SrCl ₂flux. It crystallizes in a rather large monoclinic cell with space group, C2/c. It is a member of a newly identified homologous series Sr _2n-1Os _nO _5n-2of compounds with n = 3, 5, 7,···. The crystal structure consists of stacks of two-dimensional slabs of perovskite-type layers alternating with one-dimensional zigzag chains of corner sharing OsO ₆octahedra. The (Os ^5+/6+) mixed valence title compound shows an antiferromagnetic-like transition at T _N∼75 K, as inferred from magnetic susceptibility data.