[en] Lithium-specific adsorption and lithium isotope separation characteristics were investigated on adsorbents, HMnO(Mg), prepared by treating MgMn₂O₄ with hydrochloric acid or hydrochloric acid-ammonium peroxydisulfate ((NH₄)₂S₂O₈) mixture solution. Lithium-specific adsorption was observed for the adsorbents with the spinel structure and magnesium extractability of more than about 90 %. The use of (NH₄)₂S₂O₈ with an appropriate concentration may reduce the degree of manganese dissolution in acid treatments while keeping high extractability of magnesium. Lithium isotope fractionation between the adsorbent thus prepared and a supernatant in equilibrium with each other was observed. The lighter isotope, ⁶Li, was found to be preferentially fractionated into the adsorbent phase and the value of the single-stage separation factor minus one was 5.4-7.4 x 10^-3 at 25degC. No apparent relation was found between acid treatment conditions and the lithium isotope effect as long as the adsorbent retained the spinel structure and had magnesium extractability more than about 90 %. (author)