Summary: A closer understanding of the causes for the variation of refractive index with change of composition has been sought in terms of certain properties of ions. The study is confined to isostructural ionic crystals. It is assumed that polarization of the anions (usually O²⁻ or F⁻) makes the principal contribution to the total refractive index effect, and that variation of refractive index is determined mainly by variable anion polarization. Polarization of the anion is controlled largely by the field in which it finds itself which, in turn, depends on the effective force of attraction of the cation: ionization potentials are used for the purpose of comparing such forces. For compounds involving pairs of 8- and 18-electron cations (Mg²⁺ and Zn²⁺; Al³⁺ and Ga³⁺; Si{4+} and Ge⁴⁺; Ca²⁺ and Cd²⁺) or pairs of transition cations (Zr⁴⁺ and Hf⁴⁺; Nb⁵⁺ and Ta⁵⁺; Mo⁶⁺ and W⁶⁺) refractive indices are higher in the crystal that contains the cation of the element with the higher ionization potential. For larger suites of cations of various types (for example, Mg²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺, radii from 0·65 to 0·80 Å.) the predicted relationship between ionization potential and refractive index holds precisely for the compounds A²SiF₆.6H₂O (A²⁺, etc.), but only in part for other compounds; this may be due to the effect of the crystal field on certain 3d transition cations.