The electroluminescence of Zn diffused diodes was studied in forward and reverse bias. Emission spectra obtained for forward biasing showed a near-edge as well as a low-energy line. In the composition range x>0.55, where the lowest conduction band minimum is at k=0, most photons are emitted in the near-edge line. Lasing action could be observed in this range, and the shortest wavelength where stimulated emission was obtained at 77°K was 6380 Å. For x<0.55, where the lowest conduction band minimum is the (100) minimum, the low-energy emission becomes dominant. The energy separation between near-edge and low-energy line varies between 0.4 and 0.47 eV at 77°K depending on composition. This suggests that the same defect is responsible for the low-energy emission in all these diodes (including GaAs and GaP). In reverse bias, near-edge emission and emission at higher energies than bandgap could be observed for x<0.55. The cutoff energy of the emission on the high-energy side was measured as a function of composition. It was found to vary in a manner similar to the variation of the energy of the (000) conduction band minimum. The external quantum efficiency of the forward bias emission drops sharply by two orders of magnitude at the composition x=0.55 when x is decreased. The efficiency of the reverse bias emission is independent of composition. © 1965 The American Institute of Physics.