The ilmenite form of (\mathrm{NaNbO}{3}) was synthesized hydrothermally. The ilmenite form of (\mathrm{LiNbO}{3}) was synthesized by ion exchanging the ilmenite form of (\mathrm{NaNbO}{3}). These transform irreversibly to the stable orthohombic form of (\mathrm{NaNbO}{3}) and the lithium niobate form of (\mathrm{LiNbO}{3}) at temperatures higher than 900 (\mathrm{K}). The energetics of the transformation was studied through differential scanning calorimetry and transposed temperature drop calorimetry. The calorimetric results show that the ilmenite form of (\mathrm{NaNbO}{3}) is metastable by (5.5( \pm 1.3) \mathrm{kJ} /) mole and the ilmenite form of (\mathrm{LiNbO}{3}) is metastable by 9.8 (( \pm 4.1) \mathrm{kJ} / \mathrm{mole}). Differential scanning calorimetry suggests that the (\mathrm{LiNbO}{3}) transformation occurs in two steps. The enthalpy difference between the ilmenite and the lithium niobate structures for (\mathrm{LiNbO}{3}) is compared with the difference in the lattice energies of the two structures predicted by H. J. Donnerberg, S. M. Tomlinson, C. R. A. Catlow, and O. F. Schirmer (Phys. Rev. B 40, 11909, 1989) on the basis of an ionic model and is in good agreement with the calculations. This small energy difference lends credence to the (\mathrm{LiNbO}{3}) defect model proposed by D. M. Smyth ("ISAF'86-Proceedings of the Sixth IEEE International Symposium on Applications of Ferroelectrics,"'p. 115, June 1986). The determination of the metastability of the ilmenite form of (\mathrm{NaNbO}_{3}) puts a limit on the tolerance factor for the stability of the perovskite structure. 1993 Academic Press, Inc.