A simple method for the preparation of polyacrylonitrile (PAN) nanofibers using a supercritical fluid technique, rapid expansion of a supercritical solution into a liquid solvent, is reported. It was found that PAN fibers having the size range of 50-300 nm in diameter were obtained when a solution of supercritical carbon dioxide and dimethylformamide mixture containing PAN was rapidly expanded into an ambient aqueous NaCl solution, at a pre-expansion pressure and temperature of 20.7 MPa and 60 degrees C, respectively. The effect of polymer concentration on the formation of nanofibers vs. nanoparticles was investigated. The morphological analysis by field emission scanning electron microscopy revealed that PAN nanofibers were exclusively obtained at higher polymer concentrations whereas aggregated nanoparticles were produced by lower concentrations of PAN in the supercritical solution. Using the PAN nanofibers as precursors, carbon nanofibers were obtained by the pyrolysis at 800 degrees C under N-2 atmosphere. (c) 2008 Elsevier B.V. All rights reserved.