SrS:Ce is an intensively investigated phosphor due to its blue‐green electroluminescence, which shows efficient blue emission after filtering. Recently reported devices based on this material have demonstrated a luminous efficiency of 1.6 lm/W. The luminescence properties of SrS:Ce,X (X=Na or Cl) have been studied on powders and thin films. It is shown that a high density of traps in SrS:Ce,X occurs. The interaction of Ce3+with traps gives rise to a phosphorescence. An energy transfer from Ce3+to traps is responsible for an observed luminescence quenching in the presence of high electric fields. Moreover, the traps are electrically active and are involved in the electroluminescence process. The observed energy transfer is proposed to be the dominant excitation mechanism of Ce3+in electroluminescence. It is demonstrated for thin films that the defect density increases with doping; therefore, the luminescence yield is already limited at doping concentrations below the onset of the concentration quenching. Thus, the prepared SrS:Ce,Cl thin films show a lower photoluminescence yield than powders. It is concluded that an undisturbed Ce incorporation into SrS thin films has not been achieved so far, although high electroluminescence efficiencies (1.6 lm/W) have been obtained. ©1995 American Institute of Physics.