Pure zirconium oxide nanocrystallites with diameters 6-140 nm are fabricated from ultrafine metallo-organic complexes by thermal hydrolysis at 120 degrees C and/or heat treatment at 125-1025 degrees C. X-ray photoelectron spectroscopy shows that effective ionic valence of Zr decreases with decreasing particle diameter. The size dependence of the ionic valences suggests that the phase transition from cubic to tetragonal occurs at an effective Zr valence of 2.0 near 3 nm in diameter and that the phase transition from tetragonal to monoclinic takes place at a critical size of 25 nm diameter with an effective Zr valence of about 2.6. (c) 2005 Elsevier B.V. All rights reserved.