An experimental investigation of the structure and crystal-chemistry of the triple perovskite BaSr 2 MgTa 2 O 9 is reported. Polycrystalline BaSr 2 MgTa 2 O 9 was synthesized via solid-state reaction and its structure quantitatively probed at 10 and 300​ K using neutron diffraction. Monoclinic (A2/m, a 0 b¯–b–) and trigonal structural models (P 3¯ c1, a–a–a–) were fit to diffraction data. The former was found to correctly reproduce experimental intensities, while the latter failed to do so. This difference is attributed to the ability of the monoclinic model to accurately reflect octahedral tilting. BaSr 2 MgTa 2 O 9 features corner-sharing MgO 6 and TaO 6 octahedra with barium and strontium cations occupying cubooctahedral holes. Barium and strontium are disordered over the A site, whereas B site cations magnesium and tantalum are ordered. Chemical substitution of strontium for barium in Ba 3 MgTa 2 O 9 induces tilting of MgO 6 and TaO 6 octahedra in two directions and geometric distortions of MgO 6 octahedra.