Cubic and tetragonally distorted lithium manganese oxide spinels (LiMnO2+x) were obtained by progressive oxidation of LiMnO2 between 300 and 400 degrees C. Orthorhombic LiMnO2 was prepared by hydrothermal treatment of either gamma-MnOOH or Mn(COOCH3)(3) . 2H(2)O with LiOH . H2O in large excess. The reversible specific capacity for LiMnO was found to be sensitive to average manganese valency changeable by the oxidizing condition. The LiMnO2+x exhibits the ferromagnetic behavior with T-c = 20-40 K and the occurrence of spontaneous magnetic moment is associated with the decrease in the development of ferromagnetic interaction accompanying the decrease in the average shortest Mn-Mn distance between Mn ions on octahedral 16d sites.