The partial oxidation of acrolein and methacrolein on Mo/V/W-mixed oxides was studied by transient kinetic investigation with a new experimental technique. Within the so-called HETKA (Homologue Exchange Transient Kinetic Analysis) experiments, acrolein (ACR) was stepwise replaced by methacrolein (MAC), and vice versa under constant reaction conditions. Conversion and yield decrease significantly when ACR is substituted by MAC. Evidence is given that the lower reaction rate with MAC is caused by the formation of coke deposits on the catalyst surface. Furthermore, a temperature programmed reaction (TP reaction) was carried out in which both aldehydes were fed at the same time. Compared with the TP reactions with only one aldehyde, the conversion and yield curves are shifted to higher temperatures. Regarding acrylic acid, the maximum yield and selectivity are not influenced by the contemporaneous presence of both aldehydes. On the contrary, the selectivity to methacrylic acid is increased significantly.