The invention relates to aqueous dispersions, comprising particles based on a magnetic iron oxide with dimensions of .ltoreq.20 nm, the surface of which is modified by the grafting of aminated groups R with a covalent bonding to the surface of said particles, wherein the isoelectronic point of particles with such a modified surface is .gtoreq.10. The invention further relates to a method for production of said aqueous suspensions and a method for modification of the surface of the particles present in said dispersions, in particular, by the immobilisation of polysaccharides such as dextrans, particularly for the formulation of magnetic compositions which may be administered in vivo and in particular for the formulation of injectable compositions of contrast agents for MRI.