Defect structures for Fe(III) in both alpha- and beta-diantimony tetroxide are explored using computer modelling techniques. A new favourable interstitial position for Fe(III) is identified, but the most energetically favourable defect reaction incorporating Fe(III) in diantimony tetroxide was found to be that in which the transition metal ion replaced an Sb(III) ion. The most favourable intrinsic defect in diantimony tetroxide is found to be an oxide ion Frenkel defect. This is considerably more favourable for the alpha form due to the asymmetric structure of this form. Incorporation of iron close to such a defect makes substitution on the Sb(III) site more favourable for the beta form. (C) 2007 Elsevier Ltd. All rights reserved.