The influence of yttrium on the microstructure and toughness of α-alumina is studied as a function of oxygen partial pressure and chemical state of the doping element. For doping amounts higher than 0 03 mol% of Y 2 O 3 , Y 3 Al 5 O 1 2 garnet precipitates and the alumina grain size is then limited by such a precipitation along grain boundaries. For doping amounts lower than or equal to 0 03 mol% of Y 2 O 3 , yttrium ions are in solid solution as defect complexes of the type (2Y x A l + mV o + nO i ) 2 ( m - n ) and segregates along grain boundaries. The stability of the defect complexes is promoted by low oxygen pressures and the grain size is limited by the yttrium segregation along grain boundaries. At high oxygen pressures, the yttrium solubility decreases and an enrichment in yttrium is observed at the sample surface. Simultaneously, an abnormal alumina grain growth appears. The alumina toughness, estimated by indentation, is improved by the presence of defect complexes at low oxygen pressures while the Y 3 Al 5 O 1 2 garnet precipitates increase the alumina brittleness.