Polyaniline and its co-polymers with o-nitroaniline have been synthesized by the chemical oxidation of aniline and o-nitroaniline in 0.22M (NH4)(2)S2O8 + 1M HCl aqueous solution. The yield of product was in the range 51.1-75.2%. Considering a close similarity between oxidation in chemical and electrochemical processes, the nature of the intermediates of polyaniline synthesis have been proposed based on cyclic voltammograms. The first stage of the polyaniline synthesis in both electrochemical and chemical oxidation of aniline was found to be the formation of C6H5-NH2+. cation-radical through the transfer of electron from 2s energy level of nitrogen atom to the electrode during electrochemical oxidation. The electronic structure of the initial components and intermediate particles of different charge and multiplicity have been calculated and the most energy suitable structures and the paths of their transformations have been determined. The mechanism of the step-by-step condensation of aniline includes one-electron aniline oxidation into corresponding cation-radical with subsequent recombination with the prior radical. The recombination proceeds according to the 'head-to-tail' type, which fact is indicated by the minimum value of the total and electronic energy equal to -205 402 and -1 204 688 kJ mol(-1), respectively. Nitroanilines by themselves condense under neither chemical nor electrochemical oxidation, although there are no reasons for it regarding their electronic structures.