An oxide of stoichiometry BiFe2O5−δ has been prepared by thermal treatments under high oxygen pressure. The crystallographic structure has been determined by neutron powder diffraction (NPD); it is defined in an orthorhombic structure (space group Pbam) with the Fe cations in octahedral and tetrahedral sites, similar to the previously reported BiFe2O4.5. Nevertheless, by Fourier synthesis a non-negligible occupancy of oxygen anions in interstitial positions has been determined, corresponding to a final stoichiometry BiFe2O4.63(1). It implies that some tetrahedral Fe environments are transformed into pseudo pyramids and thus, some Fe cations are oxidized to a 4+ oxidation state. We have confirmed the mixed valence of Fe by Mössbauer and x-ray-absorption spectroscopy obtaining ∼Fe3.2+ by both techniques. In addition, the magnetic structure below the magnetic ordering transition, around TN = 250 K, has also been determined by NPD. It is antiferromagnetic with a propagation vector of k = (1/2,1/2,1/2) and ordered magnetic moments of 4.6(1) μB at Fe in octahedral positions and 3.7(1) μB at Fe in pyramidal/tetrahedral positions.