A VMgO catalyst has been characterized using a battery of both in situ and ex situ techniques in order to gain insight into the relationship between the state of the catalyst surface and the catalytic performance observed during the oxidative dehydrogenation of n-butane. It has been found that treating the catalyst under oxygen-rich or oxygen-lean atmospheres markedly changes the activity, selectivity, and rate of catalyst deactivation. The performance of the catalyst under both types of atmospheres is also notably different. A certain degree of catalyst reduction is desirable for this reaction, because it increases the selectivity to dehydrogenation products by limiting the amount of oxygen available at the catalyst surface. However, the performance of the catalyst under oxygen-depleted reaction atmospheres is not stable, the main cause for catalyst deactivation being the formation of coke. An overall analysis of the observed effects is proposed on the basis of a redox mechanism.