Trap characteristics of zinc sulfide phosphors are studied as a function of chemical composition. The simplest phosphor systems are found to have simple, single‐peaked glow curves. They contain only activators and coactivators in pure sulfide base materials. Coactivators are defined as impurities necessary to stabilize the activators in the zinc sulfide lattice. They are found to exert a major influence upon trap characteristics. The trap depths are found to be 0.37 electron volt (ev) for Cl−, Br−, and Al3+, 0.51 ev for Sc3+, 0.62 ev for Ga3+, and 0.74 ev for In3+ as coactivators in . Additional glow peaks and traps are produced by oxygen and by the killers cobalt and nickel. The formation of mixed crystals with cadmium sulfide or zinc selenide generally results in a shift of the glow curves toward lower temperatures.