FeSbO4 catalysts used for selective propene oxidation/ammoxidation have been examined using a combination of XPS and TPD before and after ammonia reduction. These data indicate that the surface has a thin "skin" which is enriched in antimony oxide. This "skin" is shown to be important for selective oxidation. This two-dimensional oxide covering can be reduced to metallic antimony after only low pressure treatment in ammonia and the metallic layer can be desorbed upon heating above 600 K. The remaining surface is then covered with an Fe-rich layer which is not so easily reduced, and which is of lower selectivity in partial oxidation to acrolein.