Abstract A dramatic change is observed in the photoluminescence spectrum of a GaN sample after annealing at a temperature of ∼835°C. The as-grown sample, and the same sample annealed at 830°C, show a single dominant donor-bound-exciton (D 0, X) emission line. After annealing at ∼835°C, the emission evolves into at least four broader emission lines. Increasing the annealing temperature to 840°C reverses the emission spectrum back to the single D 0, X line. Repeating the 835°C anneal again produces the evolved spectrum. We propose a model that can account for all of the spectral lines that are generated by the 835°C anneal. We also suggest some possible explanations for the dramatic changes that occur in the emission spectrum when the annealing is carried out over small temperature intervals.