A reaction making it possible to obtain Co3O4 in the form of spheres with a narrow size distribution and diameters of ca. 0.2-0.3 mu m, depending on the temperature used, is described. The starting reagent is a trivalent cobalt complex salt (Na3Co(NO2)(6)) which is decomposed in the presence of ammonium buffers. Precipitation begins from initially homogeneous solutions by separation of Co3O4-CoOOH mixtures. Upon heating, the latter compound progressively disappears transforming to Co3O4 by a phase change supported by Co(II) cations which also arise from the starting complex. The end spheres are initially formed by coagulation-aggregation of much finer particles, and are thereafter consolidated during the phase transformation and contain an extended system of interconnected micropores.