Magnetic nanoparticles are of great interest in developing new drug delivery systems and in medical diagnostic tests. The stabilization of nanoparticles of magnetite coated with a biocompatible polymer is of particular interest. The adsorption of novel triblock copolymers with end blocks comprised of copolymers of ethylene oxide (EO) and anchor blocks comprised of: (i) a carboxylic acid/urethane center block, (ii) a tricarboxylate end group, and (iii) a cationic PEI end block was studied on particles of magnetite, Fe3O4 approximately 10 nm in diameter in aqueous media. Adsorption via the anchor blocks led to the formation of a brush layer that sterically stabilized the particles. The extension of the brush layer from the particle surface was modeled using a model based on star polymer molecules and the resulting steric contribution to the modified DLVO theory was sufficient to stabilize the particles in aqueous buffer for periods of weeks in some cases. Particle size of the bare magnetite was measured by electron microscopy while the size of the particles with adsorbed block copolymers was measured by dynamic light scattering. The particle surface area was measured by the BET method and the amount of bound polymer was measured by thermal gravimetric analysis. The zeta potential was measured with an electrophoretic dynamic light scattering instrument. The effect of an external magnetic field on the colloid stability of the particles was also accounted for in the DLVO theory. The block copolymer composition was varied and the resulting effect on particle stabilization will be discussed.