The cucial point in selecting appropriate host materials and dopants for electroluminescent devices is the knowledge of the ionization energies of impurity ions from which an efficient emission may be obtained. RE ions represent a relatively simple system of dopants in wide bandgap materials. Due to the screening of the 4fq shell by the external 5s and 5p shells, all electronic and magnetic properties are either identical or close to those of free ions, and depend rather weakly on lattice ionicity, lattice parameters, and stucture. It was carefully tested that the energies of intra-ion and ionization transitions of a given RE ion are similar or nearly identical in crystals with the same ligand and independent of lattice "details" [1,2].Recently, we have shown that for Eu, Tm, and Yb the RE 2 +/3+ energy level is located in the forbidden gap of ZnS [3]. The energies of appropriate PI (charge transfer) transitions calculated basing on the concepts of Jorgensen's RESPET model [1,4] coincide very well with the experimental values [5][6][7]. In this model, after correction for multiplet splitting (ΔΕΜC) of the initial and final states, the ionization energy depends linearly on " q'', i.e. on the number of electrons on the 4f shell of the RE ion. ΔΕΜC can be easily calculated since the appropriate Racah and Lande parameters were given by Carnall et al. [8]. The linear term (E -A) in q approximates three important contributions to RE ionization energy:-the regular Z-and q-dependent increase due to the contraction of the RE ionic radius, *This work was supported by CPBP 01.12.(247)