The preparation of hydrotreating catalysts by means of alumina impregnation with ammonia metatungstate, sodium tungstate and ammonium tungstate solutions, was studied. The use of several precursors, concentration levels, impregnation times and pH determined the presence of different polymeric or monomeric species in the impregnating solutions. Such species led to different tungsten distributions in the support. The impregnation with ammonium metatungstate solutions led to egg shell profiles, owing to pore blockage of the support, which results from the size exclusion of metatungstate ions. In the impregnations performed with solutions of sodium or ammonium tungstate, the resultant profiles were flat as a result of the presence of WO42- ions in solution, which, owing to its small size, penetrated deep into the spheres. The experimental tungsten concentration profiles were compared to those obtained with a theoretical model previously developed for this particular purpose. Coincidence between both was observed in profiles obtained from sodium tungstate solutions, which did not occur in results obtained from ammonium metatungstate solutions because of the obstruction mentioned above. The effect of concentration and contact time on the resultant profile shape was also studied. The profile shape obtained by impregnation with ammonium metatungstate or sodium tungstate solutions did not vary when solution concentrations were increased. When longer impregnation times were used, an increase of the total tungsten concentration was detected, but without changes in the profile type obtained. In impregnations with metatung-state solutions, little penetration of tungsten towards the sphere centre was observed, owing to blockage of alumina pores. Characterization studies on impregnated and calcined solids showed the predominant presence of the WO42- monomeric species, regardless of the precursor used in the impregnating solution.