Abstract BaTiAl 6O 12 crystallizes in the orthorhombic system with unit-cell dimensions a = 4.862(1), b = 7.136(2), c = 13.598(3) Å and space group Pnnm with z = 2. The structure was solved by Patterson and Fourier methods. Of the 714 unique reflections measured by counter techniques, 253 with I > 3 σ( I) were used in the least-squares refinement of the model to a conventional R of 0.042 ( R w = 0.037). The structure of BaTiAl 6O 12 consists of octahedra (mixed Ti and Al occupancy) and tetrahedra (all Al) in a three-dimensional array forming tunnels in which barium ions are located. The octahedra form ribbons or strings by edge-sharing to one another, and are corner-shared to tetrahedra. The corner-shared array of tetrahedra links the ribbons of octahedra. Ba 3TiAl 10O 20 crystallizes in the monoclinic system with unit-cell dimensions a = 15.631(4), b = 11.373(2), and c = 4.981(1) Å, β = 107.77(2)°, and space group C2 m with z = 2. The structure was solved by Patterson and Fourier methods and of the 2909 unique reflections measured by counter techniques, 1561 with I > 3 σ( I) were used in the least-squares refinement of the model to a conventional R of 0.046 ( R w = 0.042). The structure consists of sheets of corner-shared tetrahedra linked by parallel ribbons of edge-shared octahedra. (The octahedra are of mixed Ti and Al occupancy.) The barium ions are located in tunnels of two different types.