The physico-chemical properties and the catalytic behaviour of mixtures of iron molybdate and antimony oxide in the selective oxidation of isobutene to methacrolein were studied with special consideration of the possibility of changes of these oxides during the catalytic reaction. The catalysts were separately prepared Fe-2(MoO4)(3) and alpha-Sb2O4 and mixtures thereof. They were characterized by BET surface area measurements, X-ray diffraction (XRD), Conventional Transmission Electron Microscopy (CTEM), Electron Probe Micro Analysis (EPMA), X-ray Photoelectron Spectroscopy (XPS), Ion Scattering Spectroscopy (ISS) and Mossbauer spectroscopy before and after the catalytic reaction. Under the reaction conditions used, no mutual contamination was detectable neither before nor after test. Pure alpha-Sb2O4 is inactive. Fe-2(MoO4)(3) is active but poorly selective. The alpha-Sb2O4-Fe,(MoO4)(3) mixtures exhibit a synergetic effect, corresponding to an increase both in the methacrolein yield and in the selectivity to methacrolein. The donor properties of alpha-Sb2O4 and the acceptor properties of the Fe-2(MoO4)(3) oxide can explain this synergism in the frame of the Remote Control theory: oxygen spillover would be emitted by alpha-Sb2O4, and migrate to Fe-2(MoO4)(3) creating or regenerating selective sites on this last phase. The beneficial effect of spillover oxygen seems to reside in its ability to keep iron in a higher oxidation state, close to Fe+3.