This study focuses on the structure–activity relationships in the reaction of ethylene with ammonia to acetonitrile in the absence of gaseous oxygen over ¿-Al2O3-supported molybdenum catalysts. Previous work has proved that this reaction is structure-sensitive and that two mechanisms for the formation of acetonitrile exist. The first mechanism is based on ammoxidation with consumption of lattice oxygen and operates on freshly calcined catalysts. The second mechanism operates without the consumption of lattice oxygen on catalysts submitted to long reaction times, independent of pretreatment, and thus, it is based on oxidative ammonolysis. By applying various physico-chemical techniques, such as XRD, HREM, XPS, and LEIS, different solid state properties of the catalyst were identified. Catalysts were analysed right after pretreatment and at different times on stream. From this a relationship was derived between the solid state property of the catalyst and its catalytic property. On freshly calcined catalysts molybdenum is present as Al2(MoO4)3which is highly dispersed on the ¿-Al2O3surface. At a loading of 10 wt% Mo, where the maximum adsorption capacity of alumina is exceeded, only 77% of the ¿-Al2O3is covered with Al2(MoO4)3, indicating that adsorption occurs at specific sites. The ammoxidation mechanism is suggested to be active on this Al2(MoO4)3structure. When the catalyst is pretreated with hydrogen, the molybdenum surface species are best described as MoO2-like. A decrease in the dispersion of approximately 50% was found upon hydrogen pretreatment. It was also shown that, after long reaction times (>24 h), a highly dispersed MoO2-like structure was formed, independent of the pretreatment, containing both Mo(IV) and Mo(VI) ions which had reached a structural and chemical equilibrium. It was concluded that the oxidative ammonolysis mechanism is operational on this structure.