Morphology transition of selenious acid doped polyaniline from nanoflakes to nanorods and nanospheres was explored by changing the selenious acid-aniline (dopant-monomer) mole ratio in the aniline polymerization. The transition of polyaniline nanospheres to nanorods occurred when the dopant-monomer mole ratio was between 1 and 0.5. The formation of polyaniline nanorods was dominant when the dopant-monomer mole ratio is 0.5. At mole ratio 0.5, nanorods were obtained with the diameter at around 150 nm. At mole ratio 1, both the nanorods and nanospheres were formed and formation of the nanosphere is favored when the mole ratio is more than 1. When the mole ratio was low (0.125-0.03), polyaniline showed flakes like morphology. The morphology transition was studied by scanning electron microscopy and the molecular structure was confirmed by X-ray diffraction, FTIR and UV-vis spectroscopy. A simple and practical route to synthesize polyaniline nanostructures was demonstrated using selenious acid as effective dopant. The mechanism governing the formation of the polyaniline nanostructures is discussed. (c) 2007 Elsevier Ltd. All rights reserved.