LiMnPO4 was synthesized from an off-stoichiometric mix of starting materials with nominal composition LiMn0.9P0.95O4-delta. Stoichiometric LiMnPO4 with particle size < 50 nm was found with X-ray diffraction even with the large overall deviation from stoichiometry in the sample, indicating that other noncrystalline compounds are present. The off-stoichiometric sample had a discharge capacity of 145 mAh/g at C/10 and similar to 100 mAh/g at 2C after a constant current constant voltage charge. Capacity retention was excellent without a significant capacity loss at 1C. The performance of LiMn0.9P0.95O4-delta could not be significantly improved by diluting the electrode with more carbon, indicating a more intrinsic kinetic limitation of the material for LiMnPO4 in contrast to LiFePO4 [Nature (London), 458, 190 (2009)]. This is further corroborated by the difference in the overpotential between the two materials. Although LiMnPO4 has a sloping voltage profile, LiFePO4 tends to have a constant overpotential at high rate. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3428454] All rights reserved.