The partial oxidation of acrolein to acrylic acid on Mo/V/W-mixed oxide catalysts has been studied by means of the isotopic exchange method SSITKA (steady-state isotopic-transient kinetic analysis). In the scope of isotopic exchanges performed with the oxygen isotopes O-16(2) and O-18(2) some new details could be gained regarding the mechanism. Acrolein exchanges its oxygen with oxygen of the solid catalyst. Additionally the kinetics of the different isotopologues is influenced by another exchange reaction of oxygen between vacant and occupied surface and bulk sites. From the SSITKA results an extended model has been deduced. So, the experimentally gained data of the individual isotopologue kinetics could be verified by simulation.