The products obtained within early stages of the oxidative polymerization of aniline in solutions of various weak organic acids or in water, and aniline oligomers produced by the oxidation of aniline and aniline-(15)N in acetic acid (0.4 M) with a limited amount of oxidant were analyzed using 1H, 13C, and 15N 1D and 2D NMR spectroscopy and 1H PFG NMR. Such products are virtually identical in all cases, according to 1H NMR. They are always a mixture of products, among which one of them is prominent. Both native and neutralized forms of the products were examined. As shown by a combination of 1H DQF COSY, 1H NOESY, 1H-(13)C and 1H-(15)N HSQC, and 1H-(13)C and 1H-(15)N HMBC spectra, both forms of this product contain an oligoaniline moiety ended mostly by phenylamino groups. In a significant amount, the chains contain--either as an inner or terminal group--an unexpected six-member ring with an oxygen-containing substituted quinoneimine structure. The most probable structure of the major product is given. The difference between the native and neutralized forms of the product was examined. It is shown that the oligomeric chains, in particular quinoneimine units of the former one, are protonated. Both forms of the product exhibit a slight paramagnetism, and contain about 2x10(-9) mol g(-1) of unpaired electron spins.