While the oxidation of acrolein on mixed oxide (Mo, V, W) catalysts shows an exceptionally high selectivity with respect to acrylic acid, the selectivity of the analogous oxidation of methacrolein is only moderate. This is due to a considerably lower oxidation rate of methacrolein compared to that of acrolein, while the undesired subsequent oxidation of methacrylic acid is of the same order of magnitude as the oxidation of acrylic acid. Comprehensive kinetic studies in particular by transient experiments have led to a sound hypothesis to explain this difference and to show ways of catalyst improvement for the conversion of methacrolein to methacrylic acid. A moderate increase of methacrylic acid selectivity could be reached by variation of the Mo/V ratio of the mixed oxide. Surprisingly the addition of phosphoric acid and Cs-acetate effected a reduction of the subsequent oxidation of methacrylic acid without affecting the methacrolein oxidation, leading to a marked increase of the selectivity toward methacrylic acid. Furthermore, the comparison of methacrolein and acrolein oxidation results in a confirmation of the hypothesis that aldehyde oxidation and subsequent oxidation of acid occur on different domains of the mixed oxide catalyst.