The anodic oxidation of 3,3'-dimethoxybenzidine and its derivatives were studied by cyclic voltammetry, voltammetry at a rotating platinum electrode, chronopotentiometry and constant potential coulometry. The exhaustive oxidation of 3,3'-dimethoxybenzidine led at the potential of the first oxidation peak and second oxidation peak, respectively, to a stable cation radical salt (characterized by ESR spectroscopy) and to a quinone-diimine. Whereas, 3,3'-dimethoxy-N,N,N,N'-tetramethylbenzidine afforded directly a dication salt owing to potential inversion 3,3'-dimethoxy-N,N,N',N'-tetraethylbenzidine gave the corresponding soluble cation radical salt. (C) 2003 Elsevier Science Ltd. All rights reserved.