Ionic composition of lithium metaniobate and metatantalate nanopowders is modified in calciumcontaining chloride melts. Investigation of the reaction products by the methods of X-ray diffractometry, Raman spectroscopy, scanning electron microscopy, energy dispersion X-ray spectroscopy and elemental chemical analysis showed that surface layer of the modified small-dimension particles of lithium niobate and tantalate powders is enriched with thecalcium metaniobates or metatantalate (CaNb2O6, CaTa2O6) formed in the result of the isomorphic heterovalent substitution reaction of lithium ions by calcium ions in molten LiCl-CaCl2 and KCl-CaCl2 mixtures at the temperatures of 700 и 750°C.