The demand for critical materials such as Li and Co increases with the demand for lithium ion batteries. Since there are no natural sources of Li and Co in Japan, it is desirable to recover these critical materials from batteries. Though LiCoO2 is mainly used as the cathode material, it could be replaced by cheaper materials such as Li(Co1/3Ni1/3Mn1/3) O-2, LiMn2O4 and LiFePO4. Recovery of these valuable metals from lithium ion batteries by hydrometallurgical processes involves complex treatments including leaching, precipitation, and solvent extraction, together with costly steps to remove less valuable metals such as Fe and Al. In this study, the separation of Li+ and Co2+ from Li+/Fe3+/Al3+ and Co2+/Fe3+/Al3+ solutions was investigated with solvent extraction using the masking effect of chelating agents. On addition of chelating agents, it was found that Fe3+ and Al3+ formed water-soluble complexes, allowing Li+ and Co2+ to be selectively extracted in the organic layer. This effect depends on the stability of extractant-metal ion complexes and chelating agent-metal ion complexes. On addition of chelating agents, Fe3+ and Al3+ formed water-soluble complexes, allowing Li+ and Co2+ to be selectively extracted in the organic layer.