The synthesis and structural characterization of 15R-, 8H-, 6H′-, and 10H-BaMnO3-x and 9R-Ba0.875-Sr0.125MnO 3 is described. Neutron powder diffraction data reveal a strong preference for the location of anion vacancies within the face-shared hexagonal layers of these phases. At high anion vacancy concentrations, Mn 2Og9-x dimer units become prevalent. Magnetic characterization indicates antiferro-magnetic order in all phases. Ordering temperatures are in the range 250 K
