The oxidative polymerization of aniline in aqueous acidic solution was carried out in the presence of a variety of organic compounds as potential traps for postulated intermediates. The polymerization was inhibited by hindered phenols and electron-rich alkenes, traps for cation-radicals. However, polyaniline was still obtained in the presence of electron-rich arenes, such as 1,3-dimethoxybenzene and 1,4-dimethoxybenzene, known as excellent receptors of nitrenium ions. Polymerization of N-phenyl-1,4-phenylenediamine was similarly carried out in the presence of potential traps. Polyaniline containing an N-phenyl group was obtained in the presence of 1,3-dimethoxybenzene and 1,4-dimethoxybenzene. Hindered phenols and 4-methoxystyrene only slightly inhibited polymerization of N-phenyl-1,4-phenylenediamine which most probably proceeded by way of the stable diarylamino radical. Copolymerization of aniline with 10 wt % of N-phenyl-1,4-phenylenediamine in the presence of these traps gave similar results to the polymerization of pure aniline. These results have led to the proposed cation-radical polymerization mechanism of aniline, in which the polymerization is a chain growth reaction through the combination of a polymeric cation-radical and an anilinium cation-radical. Step growth character is also present when a polymeric aminium cation-radical end combines with a diarylaminoended polymer. The copolymerization of N-phenyl-p-phenylenediamine can also occur by reaction of aniline cation-radical with a polyarylamine radical. The nitrenium mechanism was further rejected by the fact that attempted polymerization of N-phenylhydroxylamine, which forms authentic nitrenium ions in acid, failed to give polymer.