Crystal structure and oxygen stoichiometry in LiMn1.5Ni0.5O4-delta, a potential lithium-battery cathode, vary with temperature, as observed in samples quenched from different temperatures and by in situ diffraction and thermogravimetry techniques. When prepared in high O-2 pressure, this cation-ordered spinel is oxygen-stoichiometric, delta = 0, space group P4(3)32. Upon heating between 650 and 680 C, increasing oxygen deficiency occurs exclusively in MnO6 octahedra and Mn-O-Mn bonds, which induces a volume increase of the 12d octahedra, a reduction of Mn as shown by X-ray absorption near-edge structure, equalization of Mn-O and Ni-O bond lengths, and disordering of Mn, Ni on octahedral sites. Hence, the transformation to space group Fd (3) over barm, shown by Rietveld refinement of variable-temperature neutron diffraction data, is a direct consequence of oxygen loss from the structure. On further oxygen loss, a second phase transformation occurs to give a cation-deficient cubic rock salt phase, delta similar to 0.65, at 950 degrees C, which loses more oxygen at higher temperatures until, at 1100 degrees C, the material is essentially a stoichiometric, single-phase cation-disordered rock salt, space group Fm (3) over barm. A second spinel phase persists in small amounts from 950 to 1100 degrees C. Differences in electrochemical behavior depend on sample preparation and correlate with the oxygen content of LiMn1.5Ni0.5O4-delta when used as a cathode in Li test cells. (c) 2008 The Electrochemical Society.