The dependence on the composition of the oxide catalyst of the rate and selectivity of the oxidative dehydrogenation of 1-butene to butadiene over Bi2O3MoO3 catalysts has been investigated. The rate appears to be mainly determined by the Bi/Mo ratio and is optimal at Bi/Mo=1. This optimal composition is not connected with the presence of a particular compound in the binary oxide system: all compounds identified so far are either less active or not active at all. The activity pattern is complicated by a sintering reaction that differs in intensity in various ranges of the composition. The selectivity is mainly decided by the occurrence of the double-bond isomerization. This is more developed at lower temperatures and in the MoO3-rich compositions. It is considerably inhibited by a high-temperature pretreatment of the catalyst. The promoter action of Bi3+ is explained by the assumption of an enhanced tendency to dehydroxylation of the surface, thereby leading to the formation of anion vacancies that serve to accept the intermediate allylic structure. Formation of this structure is further assumed to be possible only if it becomes p bonded to Mo, thus explaining the advantage of the simultaneous presence of Bi and Mo.