Lithium manganese oxides spinels were synthesized under various synthesis conditions and their structures were determined by TOF neutron powder diffraction measurements. Oxygen vacancies exist for the samples synthesized above 800 degreesC, and the amount of vacancies was found to be sensitive to the synthesis conditions. The nearly stoichiometric spinels were synthesized at 750 degreesC in O-2 followed by heating at 470 degreesC, The transition temperatures of the cubic-orthorhombic transitions and the magnetic properties varied with the compositions, The stoichiometric spinel showed no cubic-orthorhombic phase transition.