Phase equilibria in the system HfO2–Y2O3–CaO were studied in the temperature range 1250° to 2850°C by both experimental methods (X‐ray phase analysis at 20° to 2000°C, petrography, annealing and quenching, differential thermal analysis in He at temperatures to 2500°C, thermal analysis in air using a solar furnace at temperatures to 3000°C, and electron microprobe X‐ray analysis) and theoretical means (development of a mathematical model for the liquidus surface by means of the reduced polynomial method). Phase equilibria were determined by the structure of the restricting binary systems. No ternary compounds were found. The liquidus was characterized by the presence of six four‐phase, invariant equilibria. Solid solutions were based on monoclinic (M), tetragonal (T), and cubic (F) modifications of HfO2; C and H forms of Y2O3; CaO; and CaHfO3that crystallized in two polymorphous modifications, namely, the cubic and rhombic perovskite