The effect of porosity in a wide range of mesopore diameters (d(p) = 20-330 Angstrom) on the dispersion and reducibility of cobalt species in mesoporous silicas is examined using, nitrogen adsorption, X-ray diffraction, thermogravimetric analysis, and in situ X-ray absorption. It is shown that modification of mesoporous silicas by cobalt via aqueous impregnation results in small Co3O4 crystallites located in the pores of silicas. The sizes of these crystallites increase with increasing mesopore diameters. In situ X-ray absorption and thermogravimetric analyses show that the reduction Of Co3O4 crystallites in hydrogen leads to CoO and Co metal particles. The porous structure of the supports strongly affects the extent of cobalt reduction. It is found that smaller particles in the narrow pores (20-50 Angstrom) are much more difficult to reduce to metal species than larger ones situated in the broad pores (> 50 Angstrom) of mesoporous silicas.