The separation of catalyst reduction and reoxidation is a two step process. Its application for oxidation reactions was investigated for the oxydehydrogenation of isobutyric acid to methacrylic acid with a H4PVMo11O40 heteropoly acid catalyst. Though, total oxidation is almost negligible an improvement in selectivity is obtained at certain operation conditions, especially at low temperatures. The conditions leading to selectivity improvement are described and related to catalyst properties. Deactivation experiments over many catalyst reduction/reoxidation cycles and UV-VIS-NIR and ESR spectroscopy showed reversibility of catalyst reduction if water vapor is present during reoxidation. Kinetic modeling of both steps with a Mars-van Krevelen model is presented. Calculated and experimental data show good agreement.