Magnetite Fe3O4 nanocrystal clusters (50-60 nm in diameter) with good dispersibility in aqueous medium were prepared by modified co-precipitation method in the presence of surfactants including sodium dodecylsulfate (SDS), trisodium citrate dehydrate (TSCD), and polyacrylic acid sodium salt (PAASS). The influence of surfactant on the formation of Fe3O4 nanocrystal clusters was studied. It was found that Fe3O4 nanocrystal clusters exhibited almost a super-paramagnetic behavior with very small coercivity and a high magnetization (Ms=44-56 emu g(-1)). Magnetite (Fe3O4) nanocrystal clusters-polyaniline (PANI) composites were further synthesized via a self-assembly process in the presence of Fe3O4 nanoclusters containing SDS as the dopant. It was also noted that the morphology, the magnetic, and the electrical properties of Fe3O4-PANI composites could be adjusted by altering the contents of Fe3O4 and SDS. (C) 2010 Elsevier B.V. All rights reserved.