Structures of the cubic lithium manganese spinels synthesized at 900 degrees C; and the tetragonal spinel synthesized by quenching from 920 degrees C into liquid N-2 were determined by rime-of-flight (TOF) neutron powder diffraction. The ionic distribution was determined for the tetragonal spinel to be [Li-1](tetra)(Mn-2)(octa)O-3.86(3). A small amount of oxygen vacancy was observed at the oxygen site. On the other hand, the structure of the cubic spinel determined at 325 K showed oxygen vacancy at 32e site with interstitial oxygen at 8b sites. The occupation parameters at the interstitial 8b site decreased with decreasing synthesis temperature from 900 degrees to 750 degrees C. On the basis of these findings a new oxygen vacancy cluster model is proposed.