Sorbents are prepared by extracting lithium ions from lithium manganese oxides with the lithium/manganese mole ratio (Li/Mn ratio) varying between 0.42 and 0.84, and their lithium isotope selectivities are investigated. All the sorbents are Li-6-specific and the values of the Li-7-to-Li-6 isotopic separation factor (S) are between 1.0040 and 1.0092 at 25 degrees C. The S value is slightly dependent on the Li/Mn ratio; it takes on the maximum at a Li/Mn ratio of 0.5 and decreases as the ratio deviates from this value. This indicates that the lithium isotope effect between the tetrahedral sorption sites of the sorbents and the external solution phase is larger than that between the octahedral sorption sites and the solution phase.