An original synthesis of cobalt-iron alloy/cobalt doped magnetite catalysts (spinel structure) has been developed, in which cobalt and iron are obtained in the metallic stale without any reducing treatment, The chemical phenomena occurring during the preparation, based upon the coprecipitation of the hydroxides in a boiling basic medium, are detailed. By heating under argon, the composition of the metallic phase can be controlled within the range ((Co0.8Fe0.20)-Fe-0) to ((Co0.95Fe0.050)-Fe-0). X-ray diffraction, thermogravimetric analyses, and X-pay photoelectron spectroscopy experiments lead to the characterization of the crystalline phases and their location in the bulk and at the surface of the catalysts. The catalytic behavior of these composites in the hydrogenation of carbon monoxide is studied, No reducing treatment is necessary for them to be active for the Fischer-Tropsch synthesis. Moreover, an efficient treatment for protecting the spinel phase against reduction and subsequent carburization is established, The ability of such catalysts to produce C-2-C-4 olefins is also discussed.