Cobalt nanoparticles were synthesized in mesoporous silica via a supercritical CO2 impregnation technique using methanol as an entrainer at temperatures from 313 to 353 K, pressures from 20 to 25 MPa, and methanol concentrations from 5 to 20 mol%. The loading of cobalt particles in the materials, as quantified by energy-dispersive X-ray spectroscopy, was found to depend strongly on methanol concentration and slightly on temperature, and was revealed to reach a maximum of 3.8 wt% after calcination at 333 K, 20 MPa, a methanol concentration of 10 mol%, and a fixed reaction time of 5 h. Repeated treatment under the optimized conditions improved the Co loading up to 6.6 wt%, where the Co particles were observed by transmission electron microscopy (TEM) to be in the silica mesopores.