The effect of the protonation on electropolymerization of aniline was elucidated theoretically. It was shown that the proton effects stabilities of p-aminodiphenylamine which is the primary species formed following electrooxidation in acidic media. The position of protonation is elucidated. Geometrical optimizations and calculated energies (strain and total energy and heat of formation) and proton affinity values predict that the p-aminodiphenylamine is preferably protonated on the amine nitrogen between the phenyl rings. It is concluded that the polymer backbone is relaxed and the strained structure is removed upon stepwise protonation during the growth of the polymer. (C) 2003 Elsevier Science Ltd. All rights reserved.