Poly(methacrylic acid) (PMAA)-capped Fe3O4 nanoparticles were prepared by coprecipitation with PMAA in aqueous solution. Fe3O4 nanoparticles were further assembled with 2-nitro-N-methyl-4-diazonium-formaldehyde resin (NDR) to form a photosensitive precursor film, by virtue of the coulombic attraction between the negatively charged PMAA surface capping agent and the cationic polyelectrolyte of NDR. Covalent bonds were formed under ultraviolet irradiation. As a result of polymer capping of the nanoparticles and covalent linkage, a highly stable multilayer structure was formed. Transmission electron micrographs and selected area electron diffraction pattern revealed the Fe3O4 nanoparticles to be approximately 8 nm in diameter with a cubic phase structure. X-Ray photoelectron spectroscopy provided evidence for the presence of Fe3O4 nanoparticles and NDR within the ultrathin films. The UV-visible spectroscopy and atomic force microscopy measurements supported the improvement of the stability of the film, which became impervious to polar solvents when the linkages between the nanoparticles and polymer changed from ionic bonds to covalent bonds. (C) 2003 Elsevier Science B.V. All rights reserved.