In the present work, a complete thermodynamic description of the Mn-Y-Zr-O system is presented, based on the experimental data on the Mn-Zr-O and Mn-Y-Zr-O systems which were carefully reviewed. In t- and c-ZrO2; the Mn ions dissolve in the form of Mn2+ and/or Mn3+. Their solubility limits in pure c-ZrO2 at 1673 K are calculated and compared with those of other metal ions. The ZrO2-MnOx phase diagram and three isothermal sections of MnOx-YO1.5-ZrO2 in air at different temperatures are calculated. Our optimization fits most of the experimental data very well. (c) 2005 Elsevier B.V. All rights reserved.