Abstract: In examining the ion exchange equilibria of H+/Li+ and Ca2+/Li+ on 001×20 resin, the corrected selectivity coefficients and the rational equilibrium constants for the two ion pairs are calculated with modified Brφnsted-Guggen-heim-Scatchard equation and Gaimes-Thomas formula respectively, with values of 63 and 1.66 for the latter. Some conclusions are obtained which provides the basis for using calcium ion as the displacing ion in the enrichment of lithium isotopes with high crosslinkage resin by chromatography.