The preparation of Ni/SiO2 catalysts by precipitation of silica sols with NH4OH is described. Nickel introduction during precipitation of silica sols did not affect significantly the silica surface area, yielding amorphous Ni/SiO2 catalysts in both calcined and reduced forms. This method of preparation results in Ni/SiO2 catalysts showing homogenous distribution of components, surface areas ca. 255 m(2) g(1), and mean size of nickel particles between 4 and 6 nm. The effects of gel aging and the addition of Ca or Ba cations on the textural, structural and catalytic properties of Ni/SiO2 catalysts were studied. Results are interpretated according to reported processes of dissolution-precipitation of silica occurring under basic conditions [R.K. ner, The Chemistry of Silica, Chapters 3 and 4, Wiley, New York, 1979]. Alkaline earth addition to the nickel/silica catalysts decreased the surface area to 100-155 m(2) g(-1). For reduced Ni/SiO2-Ba and Ni/SiO2-Ca catalysts, the X-ray diffraction (XRD) peak of (111) planes of metal nickel crystals was identified. Several XRD peaks not corresponding to any Ni, Si and Ba/Ca containing crystalline phase reported in the ICDD database were revealed. They were assigned to new crystalline phases including Ba and Ca in the second coordination sphere of nickel. Hydrogen adsorption on these new phases was stronger than on metal nickel. Variation of hydrogen chemisorption properties and reaction rate for benzene hydrogenation were observed with alkaline earth addition.