The study by electron diffraction and high resolution electron microscopy of several samples of the system La 0.5 Ca 0.5 MnO 3−δ , apart from strikingly confirming the correctness of the orthorhombic perovskite-related unit cell of a≈a c √2, b≈a c √2, c≈2a c for 0≤δ≤0.25, points to the existence of three dimensional multitwinning which gives rise to an apparently double cubic cell. For δ=0.5, the ordering of the anionic vacancies leads to a new material isostructural to Sr 2 FeO 5 and unit cell a=5.361(3), b=16.463(9), c=5.340(3) Å, space group Ibm2. The magnetic characterization of the three compositions has been carried out and related to the structure.