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The partial oxidation of unsaturated aldehydes such as acrolein and methacrolein to the corresponding carboxylic acids is of big importance for the chemical industry. The heart of these production plants is the oxidation catalyst. Acrylic acid is produced via the selective oxidation of acrolein on Mo/V/W mixed oxides with an excellent catalytic performance (S > 90 %, X > 95 %, life time >3 a). Under the same conditions the conversion of methacrolein to methacrylic acid has a poor catalytic performance (S < 35 %, X ≈ 40 %). To this day the reason for this is unclear. Hence, for the partial oxidation of methacrolein a heteropoly acid catalyst is used with a good performance (S ≈ 88 %, X ≈ 62 %), but unfortunately the lifetime is unsatisfactory (<1 a). Therefore, we have studied the gas and surface mechanism of the partial oxidation of acrolein to acrylic acid on mixed oxides with the help of in-situ experiments. The learning outcome of this study will help us to find a rational way to come up with a new catalytic system for the unsatisfactory partial oxidation of methacrolein to methacrylic acid and are summarised in a new extended mechanism. [ABSTRACT FROM AUTHOR]