A process for producing the precursor of a precipitated catalyst for the sub.3 synthesis, based on Fe, Al, Co and K oxides, which comprises the steps of:(A) preparing an aqueous solution of water-soluble compounds of Fe, Al and Co--adding iron as a water-soluble ferrous salt of an organic acid--and causing, by addition of an alkaline precipitating solution, the co-precipitation of the corresponding (Fe, Al and Co) hydroxides, which still contain iron in the ferrous form;(B) oxidizing the hydroxide suspension prepared according to point (A) until divalent iron and trivalent iron are in a ratio to each other which ranges from 0.20 to 0.50;(C) operating an alkaline doping with a potassium compound and heating in an inert atmosphere up to above 850.degree. C.