A mathematical model of the liquidus surface of the ZrO2 ― Y2O3 ― La2O3 phase equilibrium diagram was constructed by the use of reduced polynomials. It was established that the liquidus surface of the ternary system includes five primary solidification fields: solid solutions based on the cubic (C) and hexagonal (H) modifications of Y2O3, cubic modification of ZrO2 with the fluorite structure (F), high-temperature cubic modification of La2O3 (X), and ordered phase La2Zr2O7 with a pyrochlor structure (Py). Three four-phase invariant equilibria of the eutectic and peritectic types exist in the ternary system.