Diodes fabricated from ZnS crystals annealed in molten zinc containing 2% aluminium have been found to yield bright blue electroluminescence under reverse bias. Comparision of the electroluminescence and cathodoluminescence spectra for two type diodes.E and C together with electron microprobe and measurements of their current-voltage characteristics and brightness-current relationship in electroluminescence lead to an explanation of the production of the blue electroluminescence which is dependenton an aluminium-rich regioniswhich provides V
zn
-Al complex] as luminescencecentres and additional donors to give a low resistivity region.