The effect of Al2O3 and Nb2O5 addition as the source of + 3 and + 5 cation dopants in yttria-stabilized tetragonal zirconia polycrystals (YTZP) was examined. At around 0.2 wt.%, over the solubility limit of Al2O3, a second phase of Al2O3 was precipitated which inhibited densification. While within the solubility limit, densification increased as the amount of Al2O3 increased. It is caused by accelerated diffusion due to oxygen vacancies, which were generated during the solid solution of Al+3 in Y-TZP. When Nb2O5 was added to Y-TZP with Al2O3 precipitates, the amount of the Al2O3 precipitates decreased and densification increased as the amount of Nb+5 increased. The experimental results are discussed with defect chemistry by aliovalent cation substitutions. (C) 2004 Elsevier B.V. All rights reserved.