First-principles plane-wave pseudopotential calculations were performed to study electronic structures and formation energies of point defects in pure and Ti-doped Al2O3. In the case of pure Al2O3, the Schottky defects were found to be most stable, and the reaction energy was comparable with experiment. As compared to the Schottky defects in pure Al2O3, however, substitutional Ti3+ or Ti4+-related defects exhibited smaller formation energies. In particular, substitutional Ti3+ ions showed the smallest formation energy in the relatively reduced atmosphere. The substitutional Ti3+ induces an electron-occupied defect level in the band gap of Al2O3, which can give rise to the electronic conductivity of Ti-doped Al2O3 observed experimentally. (C) 2004 Elsevier B.V. All rights reserved.