Iron oxide nanoparticles are used in vivo as contrast agents in magnetic resonance imaging. Their widely used polymer coatings are directly involved in their biocompatibility and avoid magnetic aggregation. As these polymer brushes also limit their tissular diffusion due to important hydrodynamic sizes, this work looks to obtain particles coated with thin layers of organic biocompatible molecules. Coating molecules were chosen depending on their fixation site on iron cores; carboxylates, sulfonates, phosphates, and phosphonates, and, among them, analogs of the phosphorylcholine. Two coating procedures (dialysis and exchange resins purification) were evaluated for hydrodynamic size, total iron concentration, electrophoretic mobility, and colloidal stability. Furthermore, a complementary test on stainless steel plates evaluated the contamination by competition of phosphonates as a rough estimation of the biocompatibility of the particles. Coating with bisphosphonates, the more interesting fixation moiety, leads to small (less than 15 nm) and stable objects in a wide range of pH including the neutrality. From stability data, the coating density was evaluated at around 1.6 molecules per nm(2). Including a quaternary ammonium salt to the coating molecule lowers their electrophoretic mobility. Moreover, this type of coating protects steel plates against contamination without significant desorption. Ah these properties allow further developments of these nanoparticles for biomedical applications.