Magnetic nanoparticles based on Fe3O4 (MNP) were coated with a silica (with thin or thick layers) prior to grafting with (3-aminopropyl) triethoxysilane (APTES) and octyltriethoxysilane (OTES) to increase their hydrophobicity and their pH stability. The synthetic conditions for the APTES grafting were chosen to achieve either mono- or polylayers of chemically attached silane on the surface. The synthesized Fe3O4@SiO2- APTES and Fe3O4@SiO2-OTES core-shell particles were characterized by different methods including XRD, TEM, FTIR-ATR, N2 physisorption, ICP AES and XPS. The stability tests showed that coating with silica shell and grafting with organosilanes considerably improve the stability of MNP at pH 3; the thicker the silica layer the less iron oxide dissolved. The MNP-based materials showed catalytic activity in the esterifications of propionic acid (PA) with n-butanol and of oleic acid (OA) with trimethylolpropane. In the PA containing medium the particles partly leach into a liquid medium contrary to the OA containing medium where they seemed to be resistant to leaching.