Pd nanoparticles were successfully immobilized in the mesopores of hierarchical porous SiO2 and TiO2 using supercritical carbon dioxide (scCO(2))-acetone solution. The hierarchical porous SiO2 with macropores and mesopores demonstrated remarkable properties as a support for noble metal nanoparticle catalysts. The nano particle precursor, palladium(II) acetate (Pd(OAc)(2)), was impregnated into the mesopores of hierarchical porous SiO2 and TiO2 microparticles using scCO(2). These samples were subsequently reduced by treatment in a stream of H-2 at 473 K. During reduction, the formed Pd nanoparticles were dispersed within the hierarchical porous SiO2, with a uniform three-dimensional distribution of nanoparticles in the mesopores revealed by TEM images. The resulting Pd@hierarchical porous SiO2 exhibited high activity for the heterogeneous catalytic hydrogenation reaction of styrene in methanol solution. The catalytic activity of the Pd@hierarchical porous SiO2 was higher than that of Pd@conventional porous SiO2 without hierarchical porosity or the Pd@hierarchical porous TiO2.