In the present work, an optimized set of Gibbs energy functions is proposed for the Y-Zr-O ternary system. We focus on the ZrO2- YO1.5 quasi-binary system, but reoptimizations of the Zr-O and Y-O binary systems are included as well. The parameters for the Y-Zr binary system were taken from a previous assessment. The ZrO2-YO1.5 system was treated as a quasi-binary section of the Y-Zr-O ternary system. The existing experimental data on the ZrO2-YO1.5 system were carefully reviewed. The related parameters were optimized using both thermodynamic data and phase diagram data. A calculated phase diagram of the ZrO2-YO1.5 system is presented. Our optimization agrees well with most experimental data. Two calculated isothermal sections of the Y-Zr-O system at 1500 and 2000 K are also included. (C) 2004 Elsevier B.V. All rights reserved.