Ion exchange reaction of lithium ions on cubic tantalic acid (C-TaA) was studied on the basis of pH titration curve for Li+ at 30, 45 and 60°C. The selectivity coefficients were plotted against equivalent fraction of lithium ions in the exchanger at a given temperature, which refers to the Kielland plot The selectivity coefficient on C-TaA was independent of the equivalent fraction of lithium ions in the exchanger up to a certain point and then decreased rapidly due to a phase transition. The results indicate that the reaction was preceded by an ion exchange mechanism for early stage and then phase transition to the hexagonal system.