This paper presents experimental data on study of aging nanostructured and tetragonal single-phased 2.5 mol % Y2O3-doped ZrO2 ceramics at 250degreesC for 1000 h in a steam atmosphere. An excellent stabilization of tetragonal zirconia polycrystalline ceramics was obtained contrary to the present literature, probably due to favorable microstructural characteristics of the materials. In situ impedance and Raman spectroscopies have been coupled to correlate the electrical properties and phase content in order to provide new insight into the mechanism of monoclinic degradation. (C) 2004 The Electrochemical Society.