Phase changes of nanocrystallized undoped and 2 mol% Y2O3-doped tetragonal zirconia polycrystals (2Y-TZP) prepared by spray pyrolysis were characterized by XRD, Raman spectrometry versus temperature in dry and humid air. 2Y-TZP powders characterized by the finest crystallites and 5 wt% OH amount are remarkably stable during cooling in dry air (DTA). This excellent stability was confirmed after 12 h annealing in dry as well as in humid air at 350 degreesC. Only a approximate to 9 wt% monoclinic zirconia was detected in those latter conditions for undoped TZP. The transformation is not greatly enhanced by humidity during annealings. The effect of tetragonal deterioration of 2Y-TZP was investigated by impedance spectroscopy at 300 degreesC up to 160 h in dry air. A small addition of Al2O3 particles inhibits the tetragonal to monoclinic transformation in dry air.