The physical characteristics of the polycojugated polymer polyaniline in its emeraldine base form (EB-PANI) doped with iodine vapour have been investigated. Iodine doping occurs by the oxidation of the benzenoid diamine units of the polymer, rather than of the quinoid diimine units. The formation of triiodide anion (13) was confirmed by the appearance of a peak at 133 cm(-1) in far-IR spectrum. The degree of doping of the polymer by 13 was determined as 8.2% by gravimetric analysis. The intensity of the electron paramagnetic resonance (EPR) signal in EB-PANI increased after doping with iodine vapour, similarly to the behaviour upon acid doping and in contrast to the behaviour upon exposure of the polymer to oxygen or 1-diphenyl-2-picrylhydrazyl (DPPH) radicals. This is attributed to differences in the acidity of the species that result from reduction of the oxidizing species involved in these reactions, resulting in subsequent acid doping of the polymer in the case of oxidation by iodine. This also results in a five orders of magnitude increase in the conductivity of the polymer upon doping with iodine. (c) 2008 Elsevier Ltd. All rights reserved.