In this paper we show that the origin of the yellow luminescence band in GaN and the green luminescence band in ZnO is best explained by a transition between a shallow donor and a deep level. High resolution photoluminescence spectroscopy measurements on GaN (0 0 0 1) films grown by gas-source molecular beam epitaxy are compared with theoretical predictions. These comparisons show that the makeup of the deep level in GaN is most likely a complex consisting of a gallium vacancy, V-Ga and a nearest neighbor donor consisting of an oxygen atom on a nitrogen site, O-N, or V-Ga-O-N. By analogy, the makeup of the deep level in ZnO is most likely a complex consisting of a zinc vacancy, V-Zn and a nearest neighbor donor consisting of a chlorine atom on an oxygen site V-Zn-Cl-O. Chlorine is suggested since it is a dominant donor in ZnO and is known to be present in the ZnO growth process. (C) 1998 Elsevier Science Ltd. All rights reserved.