Chemical modifications of the 12-phosphomolybdic acid by substitution of a Mo atom by a V in the so-called Keggin primary structure and/or by introduction of counter-cations (NH4+, CS+) in the secondary structure were studied. The effects of the proportion of ammonium salt in cesium-ammonium salts mixtures and of different preparation methods were also investigated. Concurrently to characterization analyses, the kinetic parameters of the Mars and Van Krevelen (MVK) model, representing well the rate of consumption of isobutane over the solids tested were determined by varying partial pressures of isobutane and oxygen. The results were used for catalysts evaluation and as a tool for the development of a new, more active and stable formulation. On the basis of the results, the role of the constituents of the catalysts are better understood. It is proposed that vanadium atoms are easily removed from the primary structure and participate in the reduction of the catalyst but have no effect on the activation of isobutane. Moreover it is shown that ammonium ions are essential to reach a high selectivity to methacrylic acid (MAA) by reducing the active phase whereas the role of cesium is to form an alkaline salt acting as a support over which the active phase is dispersed and stabilized. (C) 2003 Elsevier B.V. All rights reserved.