We report on a microscopy and diffraction study of commercially available Cu-doped ZnS powders, along with an optical spectroscopy study of electroluminescent lamps fabricated with these powders. The results allowed us to identify a correlation between the microstructure and the optical properties (i.e. photoluminescence) of the ZnS powders. It is found that the best powders for EL lamps are those displaying a prevalent cubic, i.e. zincblend, structure, while the poor performances of the mixed phase (cubic + hexagonal) are tentatively ascribed to the quenching of luminescence due to defects (stacking faults) introduced into the cubic structure and resulting in a considerable amount of hexagonal phase, as detected by XRD. The mostly cubic powders have been selected to realize EL lamps. The optical properties of these lamps have been investigated by photoluminescence and electroluminescence spectroscopies with the aim to identify the degradation mechanisms leading to a decrease of brightness. The quenching of electroluminescence is primarily ascribed to a deterioration of the electrical contacts. However, the reduced brightness in photoluminescence spectra seems to indicate that additional quenching of luminescence is induced by transformations at the emitting layer of the lamps. This degradation is tentatively ascribed to the presence of voids in the emitting layer, which might induce, acting as traps for moisture, transformations in the optical and electrical properties of the transparent electrode (ITO).