Four types of orthorhombic olivine LiMPO4 (M:Co, Fe, NI and Mn) were investigated as high-voltage cathode active materials for Lithium secondary batteries. LiCoPO4 exhibited the highest 4.8 V discharge plateau of 100 mAh/g versus Li/Li+ after initial charging to 5.1 V and its energy density was comparable to that of layered rocksalt LiCoO2 (120 mAh/g x 4V = 480 mWh/g). In contrast, the, open circuit voltage profile of LiFePO4 was 160 mAh/g on the 3 V plateau. The Co2+ state in LiCoPO4 was confirmed using HRXRF.