Summary: The transformation of manganite during heating has been studied by single crystal X-ray diffraction methods. It is observed that manganite transforms into pyrolusite keeping its original axial directions parallel to those of the transformed phase. Pyrolusite then transforms into Mn₂O₃ with its [001] ‖ [110] or [100] of Mn₂O₃; [100] ‖ [1¯10] or [010] of Mn₂O₃ and [010] ‖ [001] of Mn₂O₃. Evidence of an intermediate product in the pyrolusite → Mn₂O₃ reaction has been found.