The crystal structure of Ba 5 (OsO 5 ) 3 Cl is described. Ba 5 (OsO 5 ) 3 Cl, M r = 1395.3, hexagonal, P6 3 cm, a = 10.928(2) , c = 7.824(5) , V = 809.2(6) 3 , D x = 6.290 Mg/m 3 . λ(MoKα) = 0.71069 , F(000) = 1170, room temperature, final R = 0.051 for 2900 observed reflections. The structure is isomorphous with Ba 5 (ReO 3 ) 3 Cl and is similar to that of standard chlorapatite; the PO 4 tetrahedra are replaced by pyramidal OsO 5 groups.