Chemical oxidative polymerization of aniline was carried out in a high viscosity reaction system, created by dissolving a high molecular weight, water-soluble polymer - polyethylene oxide - into the polymerization reaction. It was found that mass transfer, which likely influences the nucleation and growth of the polymerization products, has significant effects on the morphology of chemically prepared polyaniline (PANI) nanostructures. One-dimensional PANI nanostructures obtained in the high viscosity system had obviously smaller sizes and aspect ratios, as species diffusion was suppressed, especially with further increased system viscosity. The growth of PANI nanostructures appeared to be heavily diffusion-limited when aniline polymerization was carried out using a moderate oxidant or at very low monomer concentrations (e.g. 0.005 M) in the high viscosity system, resulting in "coral reef"-like PANI clusters. Promoting mass transfer for the polymerization carried out under very low monomer concentration in the high viscosity system was beneficial for preparation of uniform PANI nanofibers. (C) 2009 Elsevier Ltd. All rights reserved.