Mo/V mixed oxides have proven themselves to be reliable for the partial oxidation of acrolein to acrylic acid. Methacrolein which is the homologue of acrolein, reacts with clearly worse selectivity and activity on this catalyst system. By means of transient investigations (TP reduction, CP reduction, CP reaction and O-18-SSITKA) under reaction conditions, it was possible to provide a contribution towards clarifying the oxygen exchange of the Mo/V mixed oxides during the reaction of both aldehydes to the corresponding carboxylic acids. In the case of both aldchydes, an exchange of carbonyl oxygen and solid oxygen occurs on the MoN mixed oxide. (C) 2002 Elsevier Science B.V. All rights reserved.