A process for the preparation of a catalyst comprising bismuth, molybdenum, iron, cerium and other promoter elements, wherein the elements in said catalyst are combined together in an aqueous catalyst precursor slurry, the aqueous precursor slurry so obtained is dried to form a catalyst precursor, and the catalyst precursor is calcined to form said catalyst, the process comprising: (i) combining, in an aqueous solution, source compounds of Bi and Ce, and optionally one or more of Na, K, Rb, Cs, Ca, a rare earth element, Pb, W and Y, to form a mixture, (ii) adding a source compound of molybdenum to the mixture to react with the mixture and form a precipitate slurry, and (iii) combining the precipitate slurry with source compounds of the remaining elements and of the remaining molybdenum in the catalyst to form the aqueous catalyst precursor slurry.