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 4.8V discharge plateau of 100mAh/g versus Li/Li+ after initial charging to 5.1V and its energy density was comparable to that of layered rocksalt LiCoO2 (120 mAh/g×4 V=480 mWh/g). In contrast, the open circuit voltage profile of LiFePO4 was 160mAh/g on the 3V plateau. The Co2+ state in LiCoPO4 was confirmed using HRXRF.