The results of a programme of research on green-emitting epitaxial diodes are reported. Diode quantum efficiencies of 0·07% DC and 0·40% pulsed have been obtained from an optimized structure. Room-temperature cathodoluminescence measurements on the bulk regions of device structures are correlated with diode performance. Major correlations are found in luminescence efficiency, light output characteristics and spectral distribution; the last is analysed in some detail to elucidate the room-temperature luminescence mechanisms. These results, coupled with the results of attempts to change the minority carrier injection ratio in the structure and the analysis of diode characteristics, have enabled the operation of our diodes to be understood in terms of abrupt junction theory.