The microstructures and optical properties of Ce,Mg:Lu3Al5O12 scintillator ceramics are investigated with particular focus on the effect of postannealing in air from 1000 to 1450 degrees C. The formation of Al2O3 clusters after annealing above 1300 degrees C is evidenced by scanning electron microscopy. The presence of this secondary phase is tentatively explained by the occurrence of Ce and Mg evaporation, proved by inductive coupled plasma optical emission spectrometry measurements, followed by defect diffusion and clustering during high temperature annealing. Meanwhile, optical investigations including absorption, X-ray induced luminescence, light yield, scintillation decay, and thermoluminescence prove the positive role of post-annealing that leads to a brighter and faster scintillation emission. This behavior is associated to the removal of oxygen vacancies occurring during such treatments. In parallel, the partial conversion of Ce3+ ions into Ce4+ is also observed as a consequence of annealings and the role of Ce4+ ions in the scintillation process is discussed.