The adsorption of alginate (Alg) onto the surface of in water dispersed Fe3O4 nanoparticles and zeta potential of alginate-coated Fe3O4 nanoparticles have been investigated to optimize the colloidal stability of Alg-coated Fe3O4 nanoparticles. The adsorption amount of Alg increased with the decrease of adsorption pH. The zeta potential of Fe3O4 nanoparticles shifted to a lower value after adsorption of Alg. The lower adsorption pH was the lower zeta potential of Fe3O4 nanoparticles became. The Alg-coated Fe3O4 nanoparticles were found to be stabilized by steric and electrostatic repulsions. Those prepared at pH 6 were not stable around pH 5, and those prepared at pH 4 became unstable at pH below 3.5. Alg of M-w 45 kDa was a little bit more adsorbed onto nanoparticles surface than that of Mw 24 kDa. An average Fe3O4 core size of 9.3 +/- 1.7 nm was found by transmission electronic microscopy. An average hydrodynamic diameter of 30-150 nm was measured by photon correlation spectroscopy. However, an average core size of 10 rim and an average hydrodynamic diameter of 38 nm were estimated from the magnetization curve of the concentrated magnetic fluids (MFs). The maximum available saturation magnetization of MFs was about 3.5 kA/m. (c) 2006 Elsevier B.V. All rights reserved.