Highly crystalline samples of LiMnPO4 and its analogs with partial substitution of Mn by divalent Mg, Cu, Zn, and Ni were prepared by hydrothermal synthesis and characterized by X-ray diffraction and infrared spectroscopy. Chemical oxidation produced two-phase mixtures of the initial phases LiMn(1-y)MyPO4 and the delithiated forms, LiyMn(1-y)MyPO4, all with the olivine structure. The extent of oxidation depended upon the quantity of oxidizing agent used and on the identity of the substituent ions. Mg, Ni, and Cu were found to increase the level of delithiation relative to that in pure LiMnPO4. Mg was also shown to reduce the tendency of the oxidized phase to absorb water. (C) 2008 The Electrochemical Society.