The behaviour of wustite (ferrous oxide) during the catalyst preparation and its activation process has been studied. The Mossbauer spectroscopy and X-ray diffraction have been used to follow wustite transformations. Inside the wustite phase two different wustite structures were found - one attributed to the nonstoichiometric wustite and the second one, to the compound close to the "stoichiometric" FeO. The relative contents of both wustites depended on the cooling rate of the melted precursor. At the initial stage of activation, at low temperatures, nonstoichiometric ferrous oxide was transformed into magnetite and a compound closer to the stoichiometric FeO. The last one decomposes further into magnetite and metallic iron.