Lithium isotope fractionation properties were studied on five kinds of inorganic ion exchangers with different ion-sieve properties. The lithium isotope separation factors were determined batchwise in a LIOH or (LiCl + LiOH) solution at 25 or 20 degrees C, respectively. The separation factor increased in the order birnessite-type manganese oxide < hollandite-type manganese oxide < spinel-type manganese oxides < cubic antimonic acid in the LIOH solution. It was dependent on the pore radius (r(is)) of the ion-sieve and was a maximum at r(is) = 0.10 nm. The difference in the separation factor with r(is) could be well explained by considering two factors; the difference in hydration circumstances of Li+ between the ion-exchanger and the solution phase, and the influence of Li+ stabilization in the solid phase.