Extraction of lithium ions from LiMn2O4 by ammonium peroxodisulfate ((NH4)(2)S2O8) was studied, and properties of adsorbents thus obtained (MnO2(Li)) were examined with a special focus on their ability to separate the lithium isotopes. Under well-controlled conditions, (NH4)(2)S2O8 was an excellent lithium ion extractant, with satisfactorily high degrees of lithium extraction while maintaining minimal dissolution of manganese. The maximum value of the Li-7/Li-6 isotopic separation factor with MnO2(Li) thus prepared was 1.014 at 25 degrees C, which was several times larger than those of commercially available organic ion-exchangers.