Cubic tantalic acid (C-TaA) was synthesized by Li+/H+ ion exchange reaction with a 3 M (mol dm(-3)) sulfuric acid solution from LiTaO3. The LiTaO3 reactant was obtained by heating a mixture of Li2CO3 and Ta2O5 with molar ratio of 1:1 at 700, 800, and 900 degrees C. The X-ray diffraction pattern of C-TaA was indexed as cubic (space group Im3), with the lattice constant a(0) = 7.75 - 7.62 Angstrom. The C-TaA product showed an exchange reaction equivalent to one proton per one Li+ in a mixed solution of lithium nitrate and lithium hydroxide. The pH titration curves of C-TaA on the Li+/H+ exchange system indicated a weak monobasic acid. C-TaA showed high uptake for lithium ions, whereas no appreciable amount of uptake was observed for other alkali metal ions in the range studied of pH 4-12. An extremely high selectivity was found for lithium ions, and the separation factor (alpha) of lithium ion to other alkali metal ions was higher than 10(3).