Co/SiO2 nanospheres with nearly perfect core-shell structure were prepared by an improved sol-gel method combined with hydrogen reduction. The products were characterized by X-ray diffraction spectra (XRD), transmission electron microscopy (TEM) and Fourier transform infrared (FT-IR) spectra. The results indicated that polyethylene glycol (PEG 8000) could act as a more efficient dispersive reagent than citric acid monohydrate (CAM). The average size of Co nanoparticles can be well controlled with optimum concentration of PEG 8000 and the average diameter of Co nanoparticles reaches about 30 nm with PEG 8000 concentration of 50 mg ml(-1). (C) 2009 Elsevier B.V. All rights reserved.