Using the methods of physicochemical analysis (XRD, petrography, scanning electron microscopy analyses) phase equilibria were firstly investigated in the ternary system ZrO2–La2O3–Gd2O3 system at 1500 ºС. It was established that in the system there exist fields of solid solutions based on hexagonal (A) modification of La2O3 and cubic with fluorite-type structure (F) and tetragonal (Т) modification ZrО2 , cubic (С) and monoclinic (M) modification Gd2O3 and ordered intermediate phase with pyrochlore-type structure lanthanum zirconate La2Zr2O7 (Py). No new phases were found. The refined lattice parameters of the unit cells for solid solutions for the systems were determined. In the zirconia-rich corner, the solid solutions based on tetragonal modification of ZrO2 are formed. The phase field T-ZrO2 is narrow and elongated (0–18 mol% CeO2) along the ZrO2–CeO2 side of the binary system. The solubility of La2O3 in the T-ZrO2 is low and amounts to ~ 0.5 mol%, as evidenced by XRD analysis results. It is worth noting that the solid solutions based on tetragonal modification of zirconia cannot be quenched from high temperatures due to low stability of T-ZrO2 under cooling with furnace conditions. The diffraction patterns recorded at room temperatures included the peaks of monoclinic phase M-ZrO2. The homogeneity field of solid solution based on A-La2O3 extends to 31 mol% Gd2O3 and 12 mol% ZrO2 in the corresponding binary systems and locates near the composition 6,7 mol % ZrO2–90 mol% La2O3–3.3 mol% Gd2O3 on the section La2O3–(67 mol % ZrO2–33 mol % Gd2O3). It should be noted that the samples with a higher lanthanum oxide content after annealing and cooling rapidly absorb water in humid air and become hydrated. Hence, according to XRD, the hexagonal A-La(OH)3 modification forms instead of the hexagonal A-La2O3 phase. The lattice parameters for A-La(OH)3 phase vary from а = 0.6513 nm, c = 0.3847 nm the sample containing 3.35 mol % ZrО2–95 mol % La2O3–1.65 mol % Gd2O3 to а = 0.6508 nm, c = 0.3847 nm in the two-phase sample (Py+А ) containing 6.7 mol % ZrО2–90 mol % La2O3–3.3 mol % Gd2O3 and to а = 0.6477 nm, c = 0.3725 nm in the three-phase sample (Py+F+А) containing 40.2 mol % ZrО2–40 mol % La2O3–19.8 mol % Gd2O3 The isothermal section of the ZrO2–La2O3–Gd2O3 system at 1500°C contains four three-phase regions (F+Py+A, F+B+A, F+C+B, T+F+Py) and ten two-phase regions (Py+A, A+F, A+B, F+B, B+C, C+F, F+Py, Py+T, T+F, Py+F).