Single-phase samples of LaBa 2 Fe 3 O 8؉w with ؊0.204w4 0.83 were synthesized, characterized by oxygen-content analyses, powder X-ray diffraction, and magnetic susceptibility, and investigated using 57 Fe Mo] ssbauer spectroscopy. At 296 K, antiferromagnetic ordering of the Fe sites was observed for samples having w40.45. When w40.0, the iron atoms occupy predominantly an Fe 3؉ , S ؍ 5/2 state, the rest being Fe 2؉ , S ؍ 2. For w+0, the intensities of the components in the Mo] ssbauer spectra correspond to random distribution of oxygen vacancies around Fe 3؉ . When 04w40.45, increasing amounts of iron atoms enter the Fe 4؉ , S ؍ 2 state. For w50.45, which takes the paramagnetic state at room temperature, the amount of Fe 4؉ increases accordingly. At 85 K, there is inconclusive evidence of disproportionation of Fe 4؉ into Fe 3؉ and Fe 5؉ in the most oxidized w ؍ 0.83 sample. The room-temperature Mo] ssbauer data show that there occurs a gradual conversion from AF to P states with increasing w up to w50.45, when all AF components simultaneously disappear. This gradual frustration of the cooperative order has three contributing factors. First, an increasing proportion of the iron moments simply becomes paramagnetic in the AF arrangement. Second, the AF interactions become weakened upon disorder. Third, the AF interaction is obviously weakened by replacement of S ؍ 5/2 with S ؍ 2. The room-temperature relationship between the average internal magnetic field and the oxygen content parameter w resembles the magnetic field vs temperature relationship for a cooperative magnetic material. The common mechanism is that both temperature and oxygen loading affect the ordered spin system. 1998 Academic Press