Through suitable etching techniques, triangular etch pits are developed on the surfaces of electroluminescent ZnS:Cu,Cl powder crystallites at densities in the range 106−108/cm2. These pits, seen by means of the electron microscope, are essentially absent in similarly etched nonelectroluminescent ZnS:Cu,Cl crystallites. Also present on the electroluminescent particles are surface striations. The etch pits appear physically related to the striations in that the pits from only on those faces bearing the striations. All triangular pits on a given face point in the same direction, which is normal to the striation direction. This direction is one of the crystallographic polar axes in sphalerite and the c axis in wurtzite. The pits have also been found in other electroluminescent compositions such as ZnS:Cu,Al; ZnS:Cu, Cl–Pb; and ZnS:Cu, Cl–Mn. The possible interpretations of the pits and striations are discussed in terms of defect structures (e.g., dislocations and stacking faults) which may be of importance to the nature of the barriers believed to reside within the volume of electroluminescent particles. Some experiments with large crystal specimens are reported.