The brownmillerite structure provides an ordered vacancy lattice into which fluorine anions should easily be accommodated, with full occupation of the vacant sites resulting in a simple perovskite structure. Low-temperature fluorine insertion reactions with the brownmillerite Sr2Co2O5 and related materials Sr2CoFeO5 and Sr2Fe2O5 have been investigated with a view to using fluorine insertion reactions to fill the oxygen vacancies in these materials, thus forming cubic perovskite-type phases with mixed 3+/4+ B cation oxidation states. The synthesis and magnetic properties of the new perovskite materials SrCoO2.5F0.5 and SrCo0.5Fe0.5O2.5F0.5 are presented here, supplemented by a consideration of Madelung energy calculations in order to explain the dissimilar fluorination behaviour of Sr2Fe2O5. (C) 2012 Elsevier Ltd. All rights reserved.