The cubic structure of yttrium oxide is stable to 1800°C. in air as indicated by petrographic, X‐ray, and differential thermal analyses. A change in lattice parameter of less than ±0.007 a.u. was observed on heating the oxide to 1800°C. The mean specific heat of Y2O3to 1600°C. was 0.13 cal. per gm. per °C. The coefficient of linear expansion to 1400°C. was 9.3 × 10−6in. per in. per °C. Compacts of Y2O3required a temperature of 1800°C. for vitrification. In equimolecular binary mixtures heated in the powdered state at 1500°C., Y2O3formed compounds with Al2O3and Fe2O3and solid solutions with ZrO2and HfO2. Y2O3did not react with CaO, MgO, or ThO2. Crystal types and unit‐cell sizes of the reaction produc