Behavior of light yield in wide range of inorganic scintillation crystals formed by isovalent substitution of anions or cations is summarized. The correlations between improvement of light yield in mixed crystals, and the ratios of ionic radii and atomic weights of the substituted atoms, as well as the bandgap diffe rence between the components of mixed crystal, have been analyzed. It is presumed that non-monotonous dependence of light yield on components ratio in mixed scintillators is called by modulation of energy structure and limitation of carrier migration length due to spatial inhomogeneity in crystal. Phenomenological approach to prediction of light yield in mixed scintillation crystals has been proposed.