The potential of the ammonia method, initially proposed for preparing stable and well-dispersed Ni/SiO2, has been explored for the preparation of new Co/SiO2 catalysts. This method consists of adding ammonia to a solution of cobalt II nitrate to obtain ammine ions and of reacting the solution with silica support before filtration, washing and drying. When superparamagnetism occurs (no remanence, cobalt particle size less than 16-20 nm), it is shown that magnetic measurements are well adapted to the characterization of Co/SiO2 catalysts (surface average diameters calculated from magnetic techniques are in good agreement with those obtained from transmission electron microscopy). Magnetic methods also give the amount of metallic cobalt in the catalyst. The different parameters of precursor preparation (order of introduction of reactants, time of contact between silica and the solution, degree of mixing, number of washings of the filtrate, concentration of reactants, surface area of the silica support) have been varied. Increasing mixing and the silica surface area from 50 to 200 m(2)g(-1) leads to a simultaneous increase of Co loading and dispersion. By comparing the extent of reduction versus reduction temperature curves, it is suggested that the cobalt phase in precursors with cobalt loading less than ca. 8 wt% consists of a two-dimensional phyllosilicate. Reduction under flowing hydrogen at 923 K leads to an almost complete reduction with surface average diameters as small as D-s =3.8 nm associated with a narrow distribution and a cobalt loading of 3.2 wt%. Highly loaded catalysts (45.6 wt%) can also be obtained by the ammonia method with D-s=20 nm. Five typical preparation recipes are given, leading, after reduction at 923 K, to Co/SiO2 catalysts with surface average diameters spanning the range of 3.8-16 nm and cobalt loadings between 3.2-45.6 wt%.