The synthesis and structure determination of 4H-BaMnO 3− x (0⩽x⩽0.35) and 4H-Ba 0.5 Sr 0.5 MnO 3− x (0⩽x⩽0.21) is described. Neutron powder diffraction data unambiguously show the anion vacancies in the oxygen-deficient phases lie in the hexagonal BaO 3− x layers, rather than the cubic layers predicted by previous structural models. The localization of anion vacancies in the hexagonal layers is discussed on the basis of the local co-ordination polyhedra of the metal cations. Low-temperature neutron diffraction data show the oxidized materials adopt antiferromagnetic ordered structures (Ba 0.5 Sr 0.5 MnO 2.99 : TN=265K). Anion-deficient phases do not achieve magnetic order suggesting the Mn(III/IV) charge disorder suppresses long-range magnetic order.