Several of the optical transitions in ZnO and GaN appear to derive from a similar origin and have considerable overlap in the energy regions where they occur. In particular the donor-acceptor pair transitions and the well known ''yellow band'' in GaN and the analogous ''green band'' in ZnO show remarkable similarities. Because of these similarities it is likely that the respective transitions in the two materials are defect related and share common mechanisms. (C) 1997 Published by Elsevier Science Ltd.