The composition of the spinel phase in the system Li-Mn-O is studied under the influence of a controlled gas atmosphere and temperature/time schedule. The existence of an extended spinel phase of composition Li[Mn2-xLix]O-4-delta (0 less than or equal to x less than or equal to 1/3) is shown. The Li/Mn ratio as well as the proportion between Mn3+ and Mn4+ is adjustable by the method of preparation. LiMn2O4-delta can be prepared in a cubic and tetragonal modification. According to magnetic and spectroscopic measurement, no simple ionic model can describe the electron structure throughout the entire series of solid solution. A partial charge delocalization takes place resulting in a reduction of the total number of unpaired electrons. At lower temperatures a spin glass behaviour develops due to the inherent frustration of the octahedral antiferromagnetic sublattice. The freezing temperature depends on the Li/Mn ratio and decreases with increasing x.