The primary radical cation intermediate of the one electron oxidation of p-anisidine into variamine blue dication has been characterized by cyclic voltammetry in the 10 kV s(-1) range at gold ultramicroelectrodes. Combined use of fast scan voltammetry with cyclic voltammetry in the usual range of scan rates allowed us to establish that the rate determining step of the chemical sequence leading ultimately to the variamine blue dication is a reaction between the electrogenerated radical cation and p-anisidine (DIM2 mechanism). The rate constant of this fast reaction (5 X 10(7) M-1 s(-1)) has been measured, yet its exact nature still remains unknown. This may correspond to the effective coupling step, or to an irreversible proton exchange between the radical cation and p-anisidine, this latter being the strongest base present in the medium. In this latter case, the coupling step occurs after this rate determining step and involves the radical thus formed with either p-anisidine or its cation radical.