The effect of the pretreatment of ZnS via different methods on the characteristics of synthesized ZnS:(Cu, Cl) electroluminescent phosphors with varying concentrations of the activator dopant (Cu) is studied. As a result of the pretreatment of ZnS with accelerated electrons and in nitrogen plasma, an increase in the emission brightness of the synthesized phosphor and a shift of the luminescence spectrum to longer wavelengths are observed. These effects are attributed with increase in content of the activator in the phosphor matrix because of the formation of extra defects in the ZnS structure during treatment. In the case of shock-wave treatment of ZnS, the generation of defects is not compensated by heat treatment during synthesis of the phosphor, and a positive enhanced-brightness effect is not attained.