Antimonic acid with the monoclinic structure, M-SbA, which had a high selectivity for the lithium ion among alkali metal ions, was prepared by extracting lithium ions from LiSbO3 through ion exchange with protons and its lithium isotope selectivity has been studied. M-SbA underwent the reversible monoclinic-orthorhombic structural change upon desorption-sorption of lithium ions while it showed the irreversible monoclinic-cubic structural change upon sorption of sodium ions. Isotopically, M-SbA was a Li-6-specific ion exchanger. The Li-7-to-Li-6 isotopic separation factor was slightly dependent on the kinds of counterion in the solution phase of the batch experiments and the maximum S value obtained at 25 degrees C was 1.025.