The Mutual Promotion Effect of Molybdates in Multicomponent Molybdate Catalysts Multicomponent molybdate catalysts (e.g. of the composition Bi2Fe1,5Co7Cr10Mo19Oy) are produced by coprecipitation, drying and calcination. These catalysts exhibit high activity and selectivity in the ammoxidation of propene to acrylonitrile. The individual molybdates (Co‐, Fe‐ and Cr‐molybdate) exhibit no selectivity in the ammoxidation of propene. Therefore a mutual promotion effect of these molybdates exists. The cause of the promotion effect is the formation of the active and selective phase from the β‐phase of Co‐molybdate by exchange of protons with ions of Fe3+ and Cr3+ from the molybdates of Fe and Cr. The structure of this catalytic active and selective phase is similar to the structure of the β‐phase of Co‐molybdate.