The coupled neutron diffraction and Mossbaüer study of the layered oxide Sr3NdFe3O9 shows for thefirst time the possibility of anionic vacancies ordering in an oxygen-deficient related Ruddlesden-Popper-(RP-) type ferrite leading to an original intergrowth of brownmillerite and K2NiF4-type layers. Theantiferromagnetic properties of this phase up to 500 K are also evidenced. These results open the routeto the synthesis of new members in the series (Sr2Fe2O5)n·SrLnFeO4 and to the understanding of theirreactivity with atmospheric water.