Insulating host materials doped with trivalent cerium show quantum efficiencies of the Ce3+emission ranging from zero to unity. Comparing optical and photoelectrical properties of a very efficient scintillator material (Lu2(SiO4)O:Ce) to those of cerium doped oxides with quenched emission, the radical differences for these materials are demonstrated to originate from the location of the cerium energy levels with respect to the host conduction band. Photoionization and subsequent nonradiative relaxation processes responsible for the luminescence quenching are discussed in a donor–acceptor model for the impurity ion and a rule for luminescence efficiency is derived, applicable to a variety of phosphor and scintillator materials. ©1996 American Institute of Physics.