Abstract The phases La x Sr 2− x Fe y Ru 1− y O 4± δ ( x=0.2–0.8; y=0.6–0.9) have been synthesized by solid-state techniques and yield tetragonal structures with I4 /mmm symmetry. The oxygen stoichiometry and high-temperature structures have been examined using diffraction techniques and in situ Mössbauer spectroscopy at temperatures up to ∼600°C. Furthermore, new reduced phases that adopt structures with Immm symmetry have been discovered. Unusual coordination numbers have been determined for the most highly reduced samples with square planar coordination evident for the B site cations. The reduced orthorhombic Immm phases were found to readily reoxidize in air to the tetragonal I4 /mmm structure at relatively low temperatures of only ∼500°C.