The compound HgGa 2S 4 has been found to fluoresce at room temperature under excitation by ultraviolet-, cathode- or X-rays. The material exhibits two emission bands. If prepared with a surplus of HgS, a yellow emission band, peaking at 5700 Å is present. With a surplus of Ga 2S 3 a red emission band, peaking at 7000 Å is formed. Activation with copper increases the intensity of the red band markedly and causes a shift to 7300 Å.