The thermal reduction of Co/Al2O3 catalysts prepared by impregnation of Co(II) nitrate generally leads to migration of cobalt ions into alumina with formation of cobalt aluminate. In order to prevent the latter, it is attempted to reduce cobalt by decomposition of supported precursor salts containing ethylenediamine (en), a procedure successfully applied earlier to nickel. Alumina is impregnated with an aqueous solution containing Co(II) nitrate and two or three equivalents of en. After drying, several cobalt compounds are detected on the support by X-ray diffraction, diffuse reflectance UV-vis spectroscopy and X-ray absorption spectroscopy: crystals of [Co(III)(en)3](NO3)3 whatever the initial en/Co stoichiometry, Co(II) and Co(III)-en complexes, grafted or not, as well as a Co(II)(OH)2-like phase. The composition of these phases reveals that dramatic evolutions in cobalt speciation have taken place in the impregnating solution, driven by the oxidation of complexed cobalt by dissolved oxygen and by the precipitation of the Co(III) salt. Decomposition of these phases in an inert gas atmosphere does not lead to metal particles exclusively, as is the case of nickel, but to a mixture of oxidic and metallic phases. A treatment in hydrogen at 500 °C completes cobalt reduction, leading to 6-20 nm metal particles. This rather broad size distribution is linked to the wide range of cobalt species on the support, which stems itself from the coordination chemistry of cobalt bonded to ethylenediamine in aqueous solutions.