High-energy ball-milling of monoclinic ZrO2 was performed in air using the planetary ball mill with a stainless steel milling assembly. Structural and microstructural changes during the ball-milling were monitored using X-ray powder diffraction, Raman spectroscopy, Mossbauer spectroscopy, field emission scanning electron microscopy and energy dispersive X-ray spectrometry. The results of line broadening analysis indicated a decrease in the crystallite size and an increase in the microstrains with the ball-milling time increased up to similar to 150 min. The results of quantitative phase analysis indicated the presence of a very small amount of tetragonal ZrO2 phase in this early stage of ball-milling. The onset of m-ZrO2 -> t-ZrO2 transition occurred between 10 and 15 h of ball-milling, which resulted in a complete transition after 20 h of ball-milling. Further ball-milling caused a decrease of the t-ZrO2 lattice parameters followed by a probable transition into c-ZrO2. It was concluded that the stabilization of t- and c-ZrO2, polymorphs at RT can be attributed to the incorporation of aliovalent cations (Fe2+, Fe3+ and Cr3+) introduced into the sample due to the wear and oxidation of the milling media. (c) 2005 Elsevier Ltd. All rights reserved.