The crystal structure of Ba 4 Os 6 O 1 8 Cl is described. Ba 4 Os 6 O 1 8 Cl, M r = 2013.97, cubic, I23, a = 9.380(1) , V = 825.3(3) 3 , Z = 2, D x = 8.104 Mg/m 3 , λ(MoK α ) = 0.71069 , F(000) = 1538, room temperature, final R W = 0.019 for 203 observed Friedel pairs. The structure is related to that of KSbO 3 and is similar to that of La 4 Os 6 O 1 9 . The La 4 O groups are replaced by Ba 4 Cl groups.