The Li+/H+ ion-exchange reaction was examined between cubic cubic acid (c-HNbO3) and rhombohedral lithium niobate (r-LiNbO3). The proton exchange of r-LiNbO3 in concentrated nitric acid occurred through an apparent eutectic mixture of c-HNbO3 and r-LiNbO3. The X-ray diffraction peaks of r-LiNbO3 simply decreased while those of c-HNbO3 increased inversely. The conversion to c-HNbO3 was almost 100%. The reverse lithium ion-exchange of c-HNbO3 occurred through a homogenous solid solution of cubic LixH1-xNbO3 between x from 0 to 0.2. A rhombohedral phase of LixH1-xNbO3 appeared x > 0.2 coexisting with the cubic phase. The maximum value of x was about 0.45 and 0.61 at 30 degreesC and 60 degreesC, respectively. The irreversible behavior of the Li+/H+ ion-exchange reaction from c-HNbO3 to r-LiNbO3 was examined by means of the H-1 and Li-7 NMR spectroscopy. The mobility of lithium ions as well as protons in Li(x)H(1-x)bO(3) was found to decrease with increasing the lithium content. The origin of the apparent irreversibility was ascribed to the decrease in the mobility of Li+ and H+ rather than the phase transition from cubic to rhombohedral.