High quality polyaniline nanofibers have been synthesized by a rapid polymerization of aniline using ammonium peroxydisulfate (APS)/Fe2+ redox initiator as the oxidant without any hard or soft templates. The addition of Fe2+ in conventional polymerization system plays an important role in changing the bulk morphologies of polyaniline from irregular particle agglomerates to nanofibers. Open-circuit potential measurements indicate that the rate of polymerization of aniline with the aid of Fe2+ ions has a substantial increase. The influences of synthetic parameters, such as the concentrations of aniline, dopant, and redox initiator, and reaction time, on the sizes and morphologies of polyaniline nanostructures have been investigated for elucidating the formation of polyaniline nanofibers. Fourier transform infrared spectrum, UV-vis spectrum, and cyclic voltammograms reveal that the molecular structures and electrochemical properties of polyaniline nanofibers do not differ significantly from that of conventional polyaniline. (C) 2010 Elsevier Ltd. All rights reserved.