Aggregates of nanophasic amorphous iron particles in the size range of 5-10 nm have been deposited on Stober's silica submicrospheres exhibiting varying degrees of surface reactivity, by an ultrasound-driven decomposition of iron pentacarbonyl in Decalin medium. The reaction of the surface-active species such as adsorbed water, silanols, and the bridging oxygens of the strained siloxane links with elemental iron and their influence on the nature of the target iron oxide nanoparticles have been examined by X-ray diffraction (XRD), transmission electron microscopy (TEM), FT-IR, BET nitrogen adsorption, and magnetic susceptibility measurements. X-ray amorphous, nanophasic iron clusters instantaneously react with untreated silica surface to form an amorphous oxyhydroxide precursor, which on crystallization under argon yields nanocrystalline Fe3O4. Silica microspheres heat treated at 450°C under argon or in vacuum (10-5 Torr) still contained enough surface-reactive species to form Fe3O4 and α-Fe2O3, respectively. Nanophasic clusters of amorphous, elemental iron could be deposited only on silica heated to 750°C. The extreme reactivity of the amorphous iron nanoparticles toward ammonia was exploited to synthesize Fe3N nanocrystals weakly adhered to silica microspheres. Pure nanocrystalline α-Fe strongly adhered to silica microspheres was obtained by the reduction of the amorphous iron precursor under flowing hydrogen. Magnetic susceptibility measurements showed all the amorphous and nanocrystalline samples to be superparamgnetic, except polycrystalline α-Fe/SiO2, which reached saturation and exhibited a hysteresis loop characteristic of ferromagnetically ordered materials. Values of interfacial magnetization coefficients extracted from the M-H data indicated a significant diamagnetic contribution from the substrate silica core, particularly in the case of crystallized samples indicative of a nonmagnetic/antiferromagnetic impurity phase formed in the interface. The formation of iron oxides on the silica surface has been discussed primarily in terms of a redox reaction between the surface σ-O-H groups and the zerovalent metal to form an oxidized iron species.