Iron-doped, ammonium/potassium salts of 12-molybdophosphoric acid (Keggin-type polyoxometalates (POMs)) were prepared, characterized and tested as catalysts for the selective oxidation of isobutane to methacrylic acid. In samples calcined at 623 K, iron was incorporated in the secondary framework of the polyoxometalate, replacing for ammonium up to a maximum amount of approximate to0.35 Fe atoms per Keggin unit. Iron was present as a partly hydrolyzed species, i.e. Fe(OH)(2)(+). Excess iron was dispersed in the form of amorphous oxide or phosphate. The calcination of the compounds at 673 K or the permanence under reaction conditions led to a partial structural decomposition of the compound; the extent of decomposition was proportional to the iron content in catalysts. (C) 2003 Elsevier B.V. All rights reserved.