The microstructure and phase transformations in the pressureless sintered composite ZrO2 with additions of 3.11 wt% high purity and ultrafine MgO powder (9.25 mol% Mg-PSZ) heat treated at 1200 C were investigated by using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction, before and after of eutectoid aging treatment at 1420 degrees C during 4 h. The phases in the as-sintered ceramics were t, c, and m, and was not evident under the experimental conditions of this work, the formation of typical disk-like shape tetragonal precipitates aligned at right angles, meanwhile the microstructure resulting in aged samples was majority monoclinic stable phase showing a banded structure which appear to be twin related. When is used MgO previously heat treated as stabilizer of ZrO2, strong differences in SEM microstructures compared to the shown by other investigators in very similar compositions have been found. (C) 2007 Elsevier B.V. All rights reserved.