An investigation of the oxygen-deficient phase SrFe1â x CoxO3â y has been carried out using ⁵⁷Fe Mössbauer spectroscopy and X-ray powder diffraction. Samples prepared under identical conditions show an increase in y as x increases. For x⩽ 0.2 the solid solutions are tetragonally distorted perovskites and show the same defect structure and novel magnetic properties which were found earlier in SrFeO3â y , but with an increase in the ordering temperature. Quenched samples show a higher degree of defect ordering than slow-cooled samples with the same Fe/Co ratio and which have a higher oxygen content. For x⩾ 0.3 there is a major structural change which the X-ray powder data indicate is to a perovskite lattice of higher symmetry, although spectroscopic evidence suggests the presence of a much more disordered defect structure. At x= 0.9 the X-ray data show the simultaneous presence of perovskite, brownmillerite, and â hexagonalâ phases, and support the contention that the â hexagonalâ structure is closely related to the brownmillerite lattice.