Electrochemical impedance spectroscopy has been used to study the oxidation of aniline at bare and polyaniline-modified platinum electrodes, Impedance measurements were used to construct a Tafel representation for the aniline oxidation. The measured (1 - alpha)n(app) value, where alpha and n(app) are the transfer coefficient and apparent number of electrons tranferred, was 0.80 for the oxidation of aniline at a a platinum electrode in 1.0 M H2SO4. The polarization resistance reached a minimum at approximately 0.90 V. We estimate the formal potential for aniline oxidation at about 0.78 V vs. an Ag/AgCl (3 M NaCl) electrode. This result indicates that the aniline oxidation, and hence polymer growth, occurs prior to the cyclic voltammetric peak potential of the aniline oxidation. It was also shown from impedance measurements that the PAn film catalyzes the aniline oxidation, and thus polymerization, reaction. When the polymer film is overoxidized, it loses its ability to get reactivated by oxidizing aniline molecules, resulting in degradation.