Antimony segregation is a common feature in Fe-Sb mixed oxides, which have been widely applied as catalysts in selective oxidation and ammoxidation reactions. This paper attempts to shed a light on the cause of such a common feature and on the nature of the antimony-enriched surface layer over FeSbO4 by means of XPS surface analysis. Single-phase FeSbO4 samples prepared by different methods were studied, and the antimony in their surface layer is a mixture of both Sb5+ and Sb3+ rather than single Sb5+. Their surface composition is close to FeSb2O6, which could be described as (FeSbO4)(Sb2O4)(delta),delta = 0.5, and it is not "Fe(II)Sb(V)(2)O-6" as suggested in literature. Fe-Sb mixed oxides with Sb/Fe > 1 (mol/mol) are mixtures of FeSbO4 and Sb2O4, and the surface of FeSbO4 grains would be a layer of (FeSbO4)(Sb2O4)(delta), delta >= 0.5. Fe-Sb mixed oxides with Sb/Fe < 1 are mixtures of FeSbO4 and Fe2O3, and the surface of FeSbO4 grains would be a layer of (FeSbO4)(Sb2O4)(delta), delta <= 0.5, but the remaining Fe2O3 would be encapsulated by a layer of FeSbO4. (c) 2005 Elsevier B.V. All rights reserved.