To better understand the factors responsible for the poor electrochemical performances of the olivine-type LiMnPO4, various experiments such as chemical delithiation, galvanostatic charge and discharge, cyclic voltamperometry, and impedance conductivity, were carried out on both LiFePO4 and LiMnPO4. Chemical delithiation experiments confirmed a topotactic two-phase electrochemical mechanism between LiMnPO4 and the fully delithiated phase MnPO4 (a = 5.909(5) angstrom, b = 9.64(1) angstrom, and c = 4.768(6) angstrom). We conclude that the limiting factor in the MnPO4/LiMnPO4 electrochemical reaction is nested mostly in the ionic and/or electronic transport within the LiMnPO4 particles themselves rather than in charge transfer kinetics or structural instability of the MnPO4 phase. For instance, the electrical conductivity of LiMnPO4 (sigma similar to 3.10(- 9) S cm(-1) at 573 K, Delta E similar to 1.1 eV) was found to be several orders of magnitude lower than that of LiFePO4 (sigma similar to 10(-9) S cm(-1) at 298 K, Delta E similar to 0.6 eV). (c) 2005 The Electrochemical Society.