Films of Co/SiO2 composites were prepared by sequential sputtering from Co and SiO2 targets in Ar. As-grown films as well as films annealed in H2/N2 atmospheres consist of hcp Co particles of uniform size embedded in the SiO2 matrix. The measured optical absorption spectra of these composites are consistent with the Maxwell-Garnett theory except for the absence of the predicted structure that arises from the interband transitions of cobalt. Composites consisting of semiconducting Co3 O4 particles embedded in SiO2 were prepared by post-growth annealing of Co/SiO2 films in air. The optical properties of these composites were found to be comparable to those of CO3O4.