The synthesis of colloidal polypyrrole-magnetite-silica nanocomposites is described. First, silica-coated magnetite particles were prepared by the aqueous deposition of silica onto ultrafine (5-20 nm) magnetite particles via controlled hydrolysis of sodium silicate, Then pyrrole was chemically polymerized using (NH4)(2)S2O8, H2O2/Fe3+/HCl, or FeCl3 oxidants in the presence of these silica-coated magnetite particles to yield colloidal dispersions of polypyrrole-magnetite-silica particles. The magnetite contents of these materials were as high as 17.5% by mass. Vibrating sample magnetometry measurements confirmed that these nanocomposites were superparamagnetic, with bulk saturation magnetization values of up to 10.9 emu g(-1). Disc centrifuge photosedimentometry was used to assess particle size and degree of dispersion, yielding weight-average particle diameters in the range 100-520 nm. Electron microscopy studies indicated a rather ill-defined particle morphology but provided some evidence for the magnetite component within the nanocomposites. Stable colloidal dispersions were obtained using the H2O2/Fe3+/HCl and (NH4)(2)S2O8, oxidants, whereas FeCl3-based syntheses yielded only flocculated particles or macroscopic precipitates under similar conditions. Solid-state electrical conductivities of nanocomposites synthesized using (NH4)(2)S2O8, H2O2/Fe3+/HCl, and FeCl3 were about 10(-3), 10(-2), and 10(-1) S cm(-1), respectively.