Shows concentration changes of refractive index in binary systems of molten salts of alkali metals containing nitrite - and nitrate - anions. Refractive indices and molar refraction of the system KNO3 - NaNO2, and NaNO3 – KNO2 affirmed the concept of additivity and linearity of the isotherms changes in the composition.

Keywords: molten salt, nitrites and nitrates of alkali metals, refractive index, refraction