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Phase equilibria in the 1/2Y2O3–SrO–1/2Fe2O3 system were systematically studied at 1373 K in air. The homogeneity ranges and crystal structure of the solid solutions Sr1−xYxFeO3−δ with 0.875 ≤ x ≤ 1 (sp. gr. Pnma) and 0.05 ≤ x ≤ 0.25 (sp. gr. Pm3 m) and Sr3−zYzFe2O7−δ with 0 ≤ z ≤ 0.25 (sp. gr. I4/mmm) were determined by X‐ray diffraction analysis of quenched samples. The structural parameters of the single‐phase oxides were refined by the Rietveld profile method. The isothermal‐isobaric phase diagram for the 1/2Y2O3–SrO–1/2Fe2O3 system at 1373 K in air is presented. The changes of oxygen content in the solid solutions Sr1−xYxFeO3−δ (0.05 ≤ x ≤ 0.25) and Sr3−zYzFe2O7−δ with 0 ≤ z ≤ 0.25 vs temperature in air were determined by the thermogravimetric analysis. Oxygen content was also calculated from the iodometric titration results. The gradual substitution of strontium by yttrium in Sr1−xYxFeO3−δ (0.05 ≤ x ≤ 0.25) leads to a decrease in the oxygen content and the mean oxidation state of iron. The average thermal expansion coefficients for Sr1−xYxFeO3−δ (0.875 ≤ x ≤ 1 and 0.05 ≤ x ≤ 0.25) and Sr3−zYzFe2O7−δ (0 ≤ z ≤ 0.25) were calculated within the temperature range 298‐1373 K in air. The phase diagram of the 1/2Y2O3 ‐ SrO ‐ 1/2Fe2O3 system (T=1373 K, Po2=0.21 atm). [ABSTRACT FROM AUTHOR]