Well-crystallized Li4Mn5O12 powder with grain size of 0.1-0.4 mu m was prepared by heating a eutectic mixture of lithium acetate (LiOAc), and manganese nitrate (Mn(NO3)(2)), in an oxygen atmosphere. The structure of Li4Mn5O12 crystallites was found to be a cubic spinel using Rietveld refinement of both neutron and X-ray powder diffraction data. We confirmed that lithium ions occupy both the tetrahedral 8a sites, and part of the octahedral 16d sites, but not the 16c sites in the space group Fd (3) over bar m, while all the manganese ions occupy the 16d sites. The lattice parameter was found to be sensitive to the synthesis temperature as a result of the variation in manganese valence. Samples prepared at 500 degrees C showed better electrode performance. A rechargeable capacity of about 135 mAh/g for the cell Li/Li4Mn5O12 was obtained in the 2.5-3.6 V range of cell voltages.