Monodisperse magnetite nanoparticles provide a more factual model to study the interface interactions between the surfactants and magnetic nanoparticles. Monodisperse magnetite nanoparticles of 7 and 19 nm coated with oleic acid (OA) were prepared by the seed-mediated high temperature thermal decomposition of iron(III) acetylacetonate (Fe(acac)(3)) precursor method. Fourier transform infrared spectra (FTIR) and X-ray photoelectron spectroscopy (XPS) reveal that the OA molecules were adsorbed on the magnetic nanoparticles by chemisorption way. Analyses of transmission electron microscopy (TEM) shows the OA provided the particles with better isolation and dispersibility. Thermogravimetric analysis (TGA) measurement results suggest that there were two kinds of different binding energies between the OA molecules and the magnetic nanoparticles. The cover density of OA molecules on the particle surface was significantly various with the size of magnetite nanoparticles. Magnetic measurements of the magnetite nanoparticles show the surface coating reduced the interactions among the nanoparticles. (c) 2006 Elsevier B.V. All rights reserved.