A novel and efficient method to produce water dispersible superparamagnetic Fe3O4 nanoparticles is described. Nanoparticles prepared by non-hydrolytic organic phase methods are subsequently functionalized with (3-glycidyloxypropyl)trimethoxysilane, a linker that prevents aggregation and is available for subsequent coupling reactions with a wide range of polymers and biomolecules. Ring opening coupling reactions were used to coat the epoxy-functionalized magnetite nanoparticles with aminated polymers (polyetheramines) or small molecules (arginine). The resulting nanoparticles, with hydrodynamic size of 13 nm, are found to be very stable over extended periods in water or PBS due to the presence of a dense stabilizer layer covalently anchored to the surface. Exceptionally high spin-lattice relaxivity, r1, values of 17 s−1 mM−1, and low r2/r1 ratios of 3.3–3.8 were exhibited in the clinical MRI frequency range, irrespective of the molecule selected for nanoparticle stabilization. As a result the dispersions are excellent candidates for incorporation into multi-functional assemblies or for use as positive contrast agent for MRI.