The transport of energy between Mn, Fe, Co, Ni, and Cu recombination centers in ac-electroluminescence of ZnS-phosphors has been investigated. The exchange of energy is explained to be due to hole migration via the valence band: during any cycle of the voltage electrons and holes are separated temporarily and recombine only after the direction of the electric field is reversed. The delay of recombinations gives rise to a change of the distribution of holes over the activators. Shortly after excitation the holes are distributed according to the capture cross sections. When the field is reversed within this period the recombinations take place in centres with high capture cross section. However when the frequency of the electric field is low and the temperature is so high that holes can be liberated by thermal energy, the holes are distributed following the FERMI-Statistics. The change of hole distribution has been studied by varying frequency, temperature and voltage.