In studying the reaction mechanism of anodic aniline oxidation the dimer p-aminodiphenylamine (ADPA) was found to be the main intermediate product in acidic solution. It is oxidized in both the free amine and the protonated form. The cyclic voltammograms have been simulated by taking into account an electrochemical reaction of both the protonated and the nonprotonated forms. Thus, it was also found that the ADPA concentration is determined by the protonation equilibrium. The oxidation product N-phenylquinone diimine (PQDI) undergoes a symproportionation reaction with ADPA forming the ADPA(.+) cation radical. The equilibrium constant for this reaction of PQDI in dimethyl sulfoxide was determined and found to be independent of the concentration of the protons in solution. The ADPA(.+) cation radical decay can be described by a rate equation based on a second-order reaction for the radical dimerization and a pre-equilibrium for the radical formation by symproportionation. The reaction mechanisms of the anodic oxidation of p-aminodiphenylamine and its chemical follow-up reactions are given.