The thermodynamic description of the ZrO 2 -GdO 1.5 -YO 1.5 system was derived based on binary assessments. One of the binaries (GdO 1.5 -YO 1.5 ) was assessed as part of this investigation, another (ZrO 2 -YO 1.5 ) was reassessed, and the third one (ZrO 2 -GdO 1.5 ) was adopted directly from a recent report in the literature. The assessments performed incorporated available experimental data on phase equilibria, calorimetric, and vapor pressure measurements. Ternary isothermal sections at 1200, 1400, and 1600°C were calculated and agreed well with phase equilibria experiments performed as part of this work. The liquidus projection, selected isopleths and a projection of the T 0 surface for the diffusionless tetragonal↔fluorite transformation in the ZrO 2 -GdO 1.5 -YO 1.5 ternary are also reported.