Temperature and field dependent Mössbauer spectroscopy has been used to characterize silica coated iron oxide nanoparticles with a core size of ~ 10 nm. Covalent attachment of oxodiperoxo molybdenum complexes to the silica surface of these particles yields an organic-inorganic hybrid nanocatalyst which is an active, magnetically separable epoxidation catalyst. Mössbauer spectroscopy identifies superparamagnetic γ-Fe2O3 particles, whereas the presence of significant amounts of Fe3O4 (≥ 5 %) can be excluded.