Ammonium salt of 12-molybdophosphoric acid (NH4)(3)PMo12O40, having the Keggin structure (polyoxometalate, POM) was prepared and tested as heterogeneous catalyst for the selective oxidation of isobutane to methacrylic acid, under both hydrocarbon-rich and hydrocarbon-lean conditions. The same compound was doped with Sb3+ ions, in an amount corresponding to 0.23 Sb atom per Keggin unit. Doping with antimony caused a decrease in the oxidation state of molybdenum, due to the electron exchange occurring between Sb3+ and Mo6+ during the calcination treatment: Sb3+ + 2Mo(6+) <----> Sb5+ + 2Mo(5+). This reduced state for molybdenum in the POM was thus stabilized even under oxidizing conditions. This effect modified the reactivity of the POM-based catalyst in the title reaction with respect to the undoped catalyst, especially under isobutane-lean conditions (i.e. 1% isobutane, 13% oxygen), thus under conditions which are considerably more oxidizing than those at 26% isobutane (isobutane-rich conditions), typically used for the selective oxidation of isobutane catalyzed by POMs. In fact, while in the absence of antimony, the oxidized catalyst was completely unselective to methacrylic acid under isobutane-lean reaction conditions, the Sb-doped catalyst reached a selectivity as high as 40-45% to methacrylic acid. On the contrary, under intrinsically reducing conditions (i.e. under isobutane-rich conditions), a reduced catalyst also developed in the reaction medium in the case of the undoped system. In this case, the advantage in selectivity gained by using a Sb-doped catalyst was much less than that obtained at low concentration of the hydrocarbon in the feed.