Comprehensive kinetic studies of the oxidation of methacrolein on mixed oxide (Mo, V, W) catalysts have been undertaken in order to find ways of catalyst improvement to achieve a high selectivity toward methacrylic acid. Steady-state kinetic experiments were carried out in a differential recycle reactor that behaves like an ideal continuous stirred tank reactor (CSTR). A kinetic model based on the scheme of Mars-van Krevelen was developed for a fair representation of the reaction rates of methacrolein conversion, methacrylic acid formation, byproduct formation, and selectivity toward methacrylic acid. For a better understanding of the kinetics, transient experiments were carried out in an apparatus to render possible sorption studies as well as transient kinetic experiments, which are powerful tools to study independently both the oxidation of the aldehydes on the catalyst in the absence of oxygen and the reoxidation of the catalyst. Steady-state kinetic data and transient experiments agree well. It could be clearly shown that the conversion of methacrolein is mainly determined by the reoxidation of the catalyst and that the selective oxidation of the aldehydes and the consecutive oxidation of the acids occur on different domains of the catalyst.