Powder neutron diffraction data have been collected between room temperature and 1000ยฐC for the oxide ion conductor La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 2.85 and the undoped parent phase LaGaO 3 . In agreement with previous studies, refinement of the data for the undoped phase showed that the cell is orthorhombic (Pbnm) at room temperature and rhombohedral (R3c) between 250 and 1000ยฐC. The structure of the doped system La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 2.85 is, however, significantly different from that of pure LaGaO 3 . The room temperature structure is monoclinic (I 2/a), and there appear to be two phase transitions between 250 and 1000ยฐC: monoclinic (pseudo-orthorhombic)Pmonoclinic (pseudo-rhombohedral)Prhombohedral. Significant changes are also seen in the GaO 6 octahedra on doping LaGaO 3 . In particular, the octahedra are substantially more distorted in the doped phase. The tilting of the octahedra is smaller for the doped phase and the degree of tilting is reduced as the temperature is raised. Possible correlation between the tilting of the octahedra and the activation energy for oxide ion conduction is presented.