Abstract Phase equilibrium was established in the Y–Fe–O system at 1100°C by varying the oxygen partial pressure from −log( P O 2 /atm)=15.00 to 0, allowing construction of a phase diagram at 1100°C for the system Y 2O 3–Fe–Fe 2O 3. In the system, two ternary compounds, YFeO 3 and Y 3Fe 5O 12, were stable and had nonstoichiometric composition, whereas YFe 2O 4 was not found to be stable. The present result was different from that of previous studies at 1200°C, in which YFe 2O 4 was stable, along with the above two ternary compounds. Lattice constants of YFeO 3 and Y 3Fe 5O 12, prepared in air by a quenching method, were determined and compared with previous values, and showed close agreement. The standard Gibbs energy changes of the reactions in the Fe–O system, Fe+1/2O 2=FeO, 3FeO+1/2O 2=Fe 3O 4, and 2/3Fe 3O 4+1/6O 2=Fe 2O 3, were determined, and the obtained values were compared with the previous values. The standard Gibbs energy changes of the reactions, Fe+1/2Y 2O 3+3/4O 2=YFeO 3, and 3YFeO 3+2/3Fe 3O 4+1/6O 2=Y 3Fe 5O 12, were calculated from the oxygen partial pressures in equilibrium.