A pure sample of a hexavalent iron compound, BaFeO 4, was decomposed at temperatures below 1200°C at oxygen pressures from 0.2 to 1500 atm. In addition to the already known BaFeO x (2.5 ≦ x < 3.0) phases with hexagonal and triclinic symmetry, two new phases were obtained as decomposition products at low temperatures. One of the new phases, with composition BaFeO 2.61 - 2.71, has tetragonal symmetry; lattice constants are a0 = 8.54 Å, c0 = 7.29 Å. The phase is antiferromagnetic with Néel temperature estimated to be 225 ± 10 K. Two internal fields observed on its Mössbauer spectra correspond to Fe 3+ and Fe 4+. In the other new phase, with composition BaFeO 2.5, all Fe 3+ ions had the same hyperfine field; it too is antiferromagnetic with a Néel temperature of 893 ± 10 K. Mössbauer data on the hexagonal phase coincided with earlier results of Gallagher, MacChesney, and Buchanan [ J. Chem. Phys.43, 516 (1965)]. In the triclinic-I BaFeO 2.50 phase, internal magnetic fields were observed at room temperature, and it was supposed that there were four kinds of Fe 3+ sites. The phase diagram of BaFeO x system was determined as functions of temperature and oxygen pressure.