New oxometallides with the formula Ba 5 Y 8−x Mn 4 O 21−1.5x (x = 0, 1) are prepared through an atmospherecontrolled solid-state reaction. Two single-phase samples with Ba/Y/Mn atomic ratios 5/8/4 (Y8) and 5/7/4 (Y7) are obtained. The crystal structures and the physical properties of the compounds are investigated by X-ray powder diffraction, magnetization, conductivity, and dielectricity measurements. The Ba 5 Y 8−x Mn 4 O 21−1.5x compound is demonstrated to be a Y-deficient solid solution. The solid solution compound Ba 5 Y 8−x Mn 4 O 21−1.5x crystallizes into tetragonal symmetry with the space group I4/m. Detailed structure analysis by Rietveld refinement of the X-ray powder diffraction data reveals that the Y vacancies occur preferentially at the Y(2) site. Thermal magnetization measurements indicate the presence of antiferromagnetic interaction between Mn ions in the compounds, and temperature-dependent resistivity measurements show that insulator-semiconductor transitions occur around 175 K and 170 K for the Y8 and Y7 samples, respectively. Strong frequency dependences of the dielectric constant are observed above ∼175 K for the two compounds.