There are enormous number of applications for rare-earth-activated materials and much oftoday’s cutting-edge optical technology and new innovations are enabled by theirpeculiar properties. In many of these applications, interactions between the rare-earth ionand the host material’s electronic states can enhance or inhibit performance and providemechanisms for manipulating the optical properties. Continued advances in thesetechnologies require knowledge of the relative energies of rare-earth and crystal bandstates so that properties of available materials may be fully understood and new materialsmay be logically developed.Conventional and resonant electron photoemission techniques were used to measure4f electron and valence band binding energies in important optical materials, includingYAG, YAlO3, and LiYF4. The photoemission spectra were theoretically modeled andanalyzed to accurately determine relative energies. By combining these energies withultraviolet spectroscopy, binding energies of excited 4f N?15d and 4f N+1 states weredetermined. While the 4f N ground-state energies vary considerably between differenttrivalent ions and lie near or below the top of the valence band in optical materials, thelowest 4f N?15d states have similar energies and are near the bottom of the conductionband. As an example for YAG, the Tb3+ 4f N ground state is in the band gap at 0.7 eVabove the valence band while the Lu3+ ground state is 4.7 eV below the valence bandmaximum; however, the lowest 4f N?15d states are 2.2 eV below the conduction band forboth ions. We found that a simple model accurately describes the binding energies of the4f N, 4f N?15d, and 4f N+1 states. The model’s success across the entire rare-earth seriesindicates that measurements on two different ions in a host are sufficient to predict theenergies of all rare-earth ions in that host.This information provides new insight into electron transfer transitions, luminescencequenching, and valence stability. All of these results lead to a clearer picture for thehost’s effect on the rare-earth ion’s electron binding energies and will motivatefundamental theoretical analysis and accelerate the development of new optical materials.