CsCdBr3 has a very unique electronic band structure [J. Kubler, private communication]. The conduction band is split up into two parts. a lower isolated and very narrow band (Delta E similar to 4000 cm(-1)) and higher lying ones. Just below the lowest conduction band we observe an absorption band of excitons localized at lattice defects. Both of them, the low-lying conduction band as well as the localized excitons, play an important role in the fast and very efficient energy transfer within the crystal. This can be shown by analyzing the decay dynamics of the fluorescence of the dopant RE-ions and of the localized and lattice excitons (excitons of the perfect lattice). Also, excitation into either the excitons or the RE-ion clearly shows an energy transfer back and forth between both of them. We deem this mechanism to be the most important one for the observed upconversion processes in CsCdBr3. (C) 2000 Elsevier Science S.A. All rights reserved.