Silica colloids embedded with superparamagnetic iron oxide nanoparticles were synthesized by combining commercial ferrofluids with the well-known Stober process. In a typical procedure, organophilic iron oxide nanoparticles were extracted from a ferrofluid and redispersed in toluene. The suspension was then added to an alcoholic medium to produce emulsion drops consisting of aggregates of iron oxide nanoparticles and toluene. When tetraethylorthosilicate was introduced into the system, it hydrolyzed and formed silica coating around each emulsion drop. The final size of silica colloids depended on the concentration of iron oxide nanoparticles and the type of solvent used for the Stober synthesis. Larger colloids were obtained at lower concentrations of iron oxide nanoparticles and in alcohols with higher molecular weights. (C) 2004 Elsevier B.V. All rights reserved.