Abstract: The capability of cation exchange memberanes, sulphomembranes of polyvinylidene fluoride, in separating lithium isotopes is determined exerimentally by single depletion and multi-stage enrichment. The separation factors,α6/7, are 0.9986-0.9967 and 1.0012-1.0018 respectively.α6/7 is effected by the degree of cross-linking of the membranes. The sulphomembrane of polyvinylidene fluoride with 15% cross-linking gives the best separation, with α6/7=0.9967.