Nanocomposites of iron oxide (Fe3O4) with a sulfonated polyaniline, poly(aniline-co-amino-naphthalenesulfonic acid) [SPAN(ANSA)], were synthesized through chemical oxidative copolymerization of aniline and 5-amino-2-naphthalenesulfonic acid/1-amino5-naphthalenesulfonic acid in the presence of Fe3O4 nanoparticles. The nanocomposites [Fe3O4/SPAN(ANSA)-NCs] were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, elemental analysis, UV-visible spectroscopy, ther-mogravimetric analysis (TGA), superconductor quantum interference device (SQUID), and electrical conductivity measurements. The TEM images reveal that nanocrystalline Fe3O4 particles were homogeneously incorporated within the polymer matrix with the sizes in the range of 10-15 nm. XRD pattern reveals that pure Fe3O4 particles are having spinel structure, and nanocomposites are more crystalline in comparison to pristine polymers. Differential thermogravimetric (DIG) curves obtained through TGA informs that polymer chains in the composites have better thermal stability than that of the pristine copolymers. FTIR spectra provide information on the structure of the composites. The conductivity of the nanocomposites (similar to 0.5 S cm(-1)) is higher than that of pristine PANI (similar to 10(-3) S cm(-1)). The charge transport behavior of the composites is explained through temperature difference of conductivity. The temperature dependence of conductivity fits with the quasi-1D variable range hopping (quasi-1D VRH) model. SQUID analysis reveals that the composites show ferromagnetic behavior at room temperature. The maximum saturation magnetization of the composite is 9.7 emu g(-1). (C) 2007 Wiley Periodicals, Inc.