Potassium/ammonium salts of 12-molybdophosphoric acid were modified by the addition of Sb3+ salt, and the chemical-physical features as well as the reactivity in isobutyric acid oxidehydrogenation of the compounds obtained were studied. All the salts prepared were monophasic and the secondary structure of the compounds was characterized by a cubic crystallographic cell. The addition of antimony led to a considerable increase in the thermal structural stability of the compounds. A second effect was on the average degree of oxidation of molybdenum, which decreased in proportion to the increase in antimony content. The results of chemical-physical characterization were interpreted by hypothesizing that antimony ions occupy a cationic position in the secondary structure of the heteropolycompound in the case of the salts containing 1 atom of potassium per Keggin unit (KU), and electronically interacts with molybdenum in the Keggin anion, leading to the formation of Mo5+ species. In the case of samples containing three atoms of potassium per KU the antimony is present in the form of dispersed salt or oxide. The modification of the molybdenum redox properties led to considerable worsening of the catalytic performance in isobutyric acid oxidehydrogenation, with a decrease in activity (for the samples with one atom of potassium per KU) and also in selectivity to methacrylic acid (for the samples with three atoms of potassium per KU). This effect was interpreted as due to the stabilization of a lower degree of oxidation of molybdenum in antimony-containing samples.