CoFe2O4-SiO2 and Fe - Co alloy - SiO2 nanocomposites were prepared by high energy ball milling of Si, alpha-Fe2O3, Co3O4 and SiO2 mixtures, followed by thermal treatment under appropriate oxidative/reductive conditions. XRD and TEM measurements carried out after 50 h of milling show highly dispersed unidentified reaction products. Further thermal treatment at 900degreesC in air leads to spherical CoFe2O4 nanocrystals (NCs) with a narrow size distribution centered around 2.5 nm, uniformly dispersed in the amorphous SiO2 matrix. Reduction of this sample in a H-2 flow at 800degreesC produces a mixture of dispersed metallic cobalt and iron silicate with traces of Fe - Co alloy NCs. Reduction of the as-milled sample, on the contrary, leads almost completely to Fe - Co alloy NCs uniformly dispersed in the SiO2 matrix and with an average particles size around 11.2 nm.