A lithium-specific sorbent, HMnO(2Mg), has been prepared by extracting magnesium ions from Mg2MnO4 through ion exchange with hydrogen ions, and its ion and lithium isotope selectivities have been studied as functions of the degree of magnesium extraction, DE(Mg). The properties as a specific-lithium sorbent revealed itself for the sorbents with a DE(Mg) value of over 50%. The maximum distribution coefficient for the lithium ion and the maximum lithium uptake from a 0.1 M LiOH solution were 14900 cm(3) g(-1) and 2.26 mmol g(-1) respectively, at DE(Mg) = 97% at 25 degrees C. Isotopically, HMnO(2Mg) was a Li-6-specific sorbent like other inorganic sorbents and organic ion exchangers so far investigated. The Li-7-to-Li-6 isotopic separation factor varied between 1.006 and 1.016 at 25 degrees C as a function of DE(Mg). High Li-6 selectivity was observed when HMnO(2Mg) did not show properties as a lithium-specific sorbent.