The reaction between aniline and p-benzoquinone in an aqueous solution of methanesulfonic acid (MSA) yields 2,5-dianilino-p-benzoquinone. It forms salts with MSA, which are conducting, the maximum conductivity being 2.6 x 10(-4) S cm(-1) when the preparation was carried out in 5-M MSA. After deprotonation to the base form, the conductivity was reduced to 10(-13) S cm-1. Paramagnetic centres (1.0 x 10(-7) mol mg(-1)) have been detected in the salt by electron paramagnetic resonance and their concentration was correspondingly reduced after the conversion to the base. The molecular structure was analysed by infrared spectroscopy and confirmed by laser-desorption-ionization mass spectrometry. The formation of polyaniline in this reaction was ruled out on the basis of chromatographic methods and UV-visible spectra. The impact of this reaction on the preparation of polyaniline, where p-benzoquinone is produced and 2,5-dianilino-p-benzoquinone may represent a major by-product, is discussed.