The Mössbauer spectra of the compounds CaFe2O4, Ca2Fe2O5, Ca2Fe1·5Al0·5O5, and Ca2FeAlO5 at temperatures above their Néel points, show quadrupole-split absorption patterns. These are assigned on the basis of point charge model calculations. The tetrahedral sites in brownmillerite are more covalent than the octahedral ones, and are preferentially occupied by aluminium.