Photoluminescence (PL) and cathodoluminescence (CL) measurements on a variety of GaN samples confirm earlier reports that epitaxial lateral over-growth (ELOG) results in improved material quality. Depth profiling of epitaxial lateral over-growth samples, reported here for the first time using variable energy electron beam excitation, shows that there is a substantial reduction in defect density away from the interface, and that the barriers to defect propagation are very effective. In addition to the normal yellow emission generally observed for GaN, we find intense blue emission, already assigned to a metastable defect, in some materials. Using a rapid scanning interferometer, we study the changes in the luminescence spectrum as a function of time and at a range of temperatures. We suggest possible explanations for the complex nature of the metastability displayed by the defect responsible for the blue band. (C) 2004 Elsevier B.V. All rights reserved.