The optical absorption band associated with the nitrogen vacancy in proton-irradiated AlxGa1-xN, for x=0.61 and 1, is investigated under isothermal furnace annealing in the temperature range of 150–350 °C. The results show that the enthalpy associated with the thermal migration process is 0.410±0.023 and 0.445±0.037 eV for Al0.61Ga0.39N and AlN, respectively. The variation of the full width at half maximum (FWHM) of the nitrogen vacancy absorption band in AlN as a function of temperature indicates that the electronic transition associated with the defect is coupled to a phonon with an energy of 47.8 meV. This phonon energy is in good agreement with the E2 phonon mode reported for AlN. A Frank–Condon shift of 1.142 eV is also estimated from the FWHM measured as a function of temperature, which is indicative of a large lattice distortion in the vicinity of the defect. © 2002 American Institute of Physics.