The phase equilibria in the Y - Ba - Fe - O system were systematically studied at 1373 K in air. The intermediate phases formed in the Y - Ba - Fe - O system at 1373 K in air were Ba3YFe2O7.5+delta and BaFe1-xYxO3-delta. The combined X-ray diffraction and electron microscopy show that Ba3YFe2O7.5+delta exhibits the monoclinic structure a = 7.976(1) angstrom, b = 5.988(1) angstrom, c = 18.408(1) angstrom, beta = 90.028 (sp. gr. P2/c), which in the first approximation can be described as the orthorhombic structure with a tiny monoclinic distortion. High-temperature XRD of BaFeO3-delta within the temperature range 298-1273 K in air demonstrates a single phase with the orthorhombic structure (sp. gr. P2(1)2(1)2) that appears below 673 K, a single phase cubic BaFeO3-delta (sp. gr. Pm3m) that forms at temperature above 1273 K, and a mixture of two phases mentioned above that coexisted within 673-1273 K. These correlate with the results obtained by TGA and dilatometry methods. The substitution of yttrium for iron in BaFe1-xYxO3-delta leads to the formation of solid solutions with the cubic structure in air within (0.075 < x < 0.125) for slowly cooled samples. The homogeneity range has extended at 1373 K towards undoped BaFeO3-delta. The projection of isothermal isobaric phase diagram for the Y - Ba - Fe - O system at 1373 K in air to the compositional triangle of metallic components has been constructed. (C) 2016 Elsevier B.V. All rights reserved.