Room-temperature Mössbauer spectra of rapidly quenched samples of nonstoicheiometric Fe_1–xO (0·053 < x < 0·109) show an envelope of overlapping resonances which, for small values of x, can be resolved into five broad Lorentzian peaks assignable to a series of Fe²⁺ quadrupole doublets and an Fe³⁺ doublet at a lower velocity. As the phase deviates further from stoicheiometry the range of site symmetries produces a more complicated envelope which cannot be resolved into the sum of a small number of Lorentzian peaks. The influence of initial temperature and of quench rate have also been studied over the full range of composition and variations correlated with the known phase diagram for the system. Samples of Fe_0·947O which had been most rapidly quenched from 1520 K into water showed negligible disproportionation during quenching. Their spectra, when compared with those predicted for various possible defect structures, suggested the presence of clusters of four vacant cation sites around a tetrahedral Fe³⁺. Changes in the spectra as the Fe³⁺ content was increased reflected the increase in the defect cluster size towards that previously proposed for Fe_0·90O (13 vacant cation sites with 4 tetrahedral Fe³⁺). Fe³⁺ was present as a quadrupole doublet and no evidence was found for electron hopping between Fe²⁺ and Fe³⁺. Spectra of the magnetically ordered oxide phases at 77 K were consistent with this interpretation.