The mode of action of the oxidation-active centres in Mo-V mixed oxides on the selective oxidation of an unsaturated aldehyde was investigated by non-steady-state methods. Various oxidation-active centres, differing in activity and selectivity, were identified by non-steady-state methods. In addition, the influence of the crystallinity of the Mo-V mixed oxides on the activity and selectivity properties were investigated. It was shown that, in contrast to crystalline samples, only X-ray-amorphous Mo-V mixed oxides contained selective oxidation centres. The measurements were used to derive a model based on the interaction of active centres with various redox properties.