Abstract The structural behavior of LaMnO 3+δ is described. One rhombohedral and three different orthorhombic X-ray patterns have been observed. The largest influence on the structure is the overall amount of Mn 4+ , but the La:Mn ratio and the temperature also have a significant influence. In the pseudobinary La 2 O 3 -Mn 2 O 3 phase diagram, LaMnO 3+δ is the only compound containing both La and Mn. This is not a line compound since a solubility range was observed on both the La-rich and the Mn-rich sides.