A novel aqueous dispersion (A) contains particles (I) based on a magnetic iron oxide and of size 20 nm or less, having a surface modified by covalently grafting amino groups R, such that the isoelectric point of the obtained surface modified particles is 10 or more. Independent claims are included for: (1) the preparation of (A) (2) (A)-containing compositions for oral or parenteral administration to humans or animals (3) a method for modifying the surface of the particles in (A), involving reacting (A) with a chemical species (E) at pH less than, to form a bond between (E) and R (4) the dispersions (A) obtained by the modification process (5) a method for further modifying the surface of the particles in (A), involving reacting (A) (in which (E) is a polysaccharide) with a chemical species (F) able to form a bond with polysaccharide and (6) the dispersions (A) obtained by the further modification process, provided that (E) is dextran linked via -NH-CH 2- moieties.
