Addition of ∼0.15-0.5 mol% acceptor oxide, Al2O3, to 3 mol% Y2O3-ZrO2 results in enhanced densification at 1350 °C. The enhancement is accounted for by a liquid phase sintering mechanism. The addition of donor oxide, Ta2O5, of 0.15-2.5 mol % at 1300-1600 °C results in the destabilization of tetragonal (t-) phase and the decrease of final density in 3 mol% Y2O3-TZP (tetragonal ZrO2 polycrystals). X-ray diffractometry (XRD) reveals that the Ta2O5-added 3 mol% Y2O3-ZrO2 contains monoclinic (m-) ZrO2 and a second phase of Ta2Zr6O17. The decreasing in final density is attributed to the increase of m-ZrO2 content. Complete destabilization of t-ZrO2 to m-ZrO2 in samples added with 2.5 mol% Ta2O5 is interpreted by the compensation effect based on donor- and acceptor-codoping defect chemistry.