The solubility, electrical conductivity, and other properties of polyaniline (PANI) are highly dependent on its oxidation state. In this work, polyaniline (PANI1) prepared by peroxodisulphate induced polymerization of aniline in acidic aqueous medium in presence of benzenediazonium chloride salt was found to exist in lower oxidation state than emeraldine form of PANI and was highly soluble in common organic solvents. This polymer was subjected to positron annihilation spectroscopic study to investigate the correlation between the oxidation state of the polymer and defect sites generated by different degrees of protonation that in turn affect its electrical conductivity. The positron annihilation lifetime data were resolved to yield a three-component fit for PANI1 subjected to different levels of protonation. The variation of positron annihilation parameters (tau(1), I-2) and Doppler broadening parameters (R, S) as a function of protonation level of the polymer indicate the dopant sites increase initially on protonation and reach a saturation value after a certain level of acidification. The lower value of electrical conductivity and the intensity of intermediate lifetime component (I-2) for PANI1 compared to PANI in emeraldine oxidation state indicate the presence of lesser number of quinoid-imine moieties that could undergo protonation and thus yield highly enriched trapping centers. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 2554-2559, 2011