The structure of the Fe 2 (Bi 0.69 Sr 2.31 )Fe 2 O 9.5À1/2d has been carried out by single crystal X-ray diffraction. Its structure can be considered as commensurately modulated with a basic structure of Fmmm symmetry and with a modulation vector q ¼ aã à þ gc à whose a and g values are, respectively, equal to 1/3 and 1. The ferrite is described as a three-fold superstructure with a % 3 a p ffiffi ffi 2 p , b % a p ffiffi ffi 2 p and c % 26 Å (space group Bbmb). It is described using a disorder model in agreement with diffuse scattering observations. The structure is built up from the intergrowth sequence between [(Bi, Sr)Fe 2 O 3.571/2d ] N complex layers, related to the rock salt (RS)-type blocks, and double perovskite layers. Large analogies are found between the present structure and the so-called 2212 modulated structures related to the high T c superconductor copper oxides or to the corresponding Fe or Co oxides; they are discussed here. A statistical disorder characterizes the [Fe 2 O 2.571/2d ] N double layer. Taking advantage of similar structural arrangements in related oxide compounds, a structural model is proposed. A description of this double layer is then given in terms of sequence of polyhedra (tetragonal pyramids, mono-caped tetrahedra and trigonal bi-pyramids), in comparison with other related modulated structures with different periodicities. The powder sample exhibits a statistical distribution of a around an average value of 0.35; the oxygen non-stoichiometry appears as the driving force for the a value, i.e. for the modulation, commensurate or incommensurate. The Mo¨ssbauer study has been performed and interpreted on the basis of the refined structure.