The absorption spectra of the oxidation products of ortho- or meta-toluidine or anisidine in acetonitrile (AN) were observed using a stopped-flow technique by mixing with AN solutions of the tris(p-bromophenyl)amine (TBPA) cation radical. During the oxidation of o-toluidine in AN, the dynamic transformation of absorption spectra could be observed in the visible region, which was assigned to the oxidized states of the benzidine dimer, o-tolidine, formed via the C-para-C-para coupling of the o-toluidine monomer. Because a decrease in the absorption of the oxidized dimer was observed in the presence of the monomers, it is inferred that the monomer contributed to the consecutive reactions of the oxidized states of the benzidine dimer in AN. In contrast, during the oxidation of o-anisidine, the formation of a similar C-para-C-para coupling dimer was hardly observed in the visible absorption spectra, while some transformations of the absorption spectra could be recorded. This indicates that the changes in the reaction sites in the case of o-anisidine are due to the substitutents. Furthermore, during the oxidation of in-toluidine or m-anisidine, no remarkable absorption was observed in the visible region, which indicated the high reactivity of the cation radicals of m-toluidine and m-anisidine. (C) 2004 Elsevier Ltd. All rights reserved.