The optical properties of GaN films doped with magnesium concentrations of the order of 1018 and 1019 cm-3 have been analyzed by picosecond time-resolved cathodoluminescence spectroscopy. Two luminescence bands in the vicinity of ∼3.45 and ∼3.27 eV are observed to increase in intensity with Mg concentration. They have been identified as related to acceptor-bound exciton and donor-acceptor pair (DAP) transitions, respectively. Time-resolved cathodoluminescence spectra, taken at low temperatures (∼4 K), show that the lifetime of the DAP peak (3.27 eV) and of what appears to be its first longitudinal-optical phonon replica at ∼3.19 eV decrease with increasing Mg concentration. These peaks differ in their recombination kinetics, strongly suggesting the existence of an underlying broad emission band which may be related to Mg. © 2008 American Institute of Physics.