The reaction of 4-bromo-N,N-dimethylaniline cation radical (DMABr . (+)) was studied using an electron-transfer stopped-flow (ETSF) method. In the ETSF method, DMABr . (+) was generated via the electron-transfer reaction between DMABr and tris (p-bromophenyl) amine cation radical. Thus, the influence of the concentration of DMABr on the reaction of DMABr . (+) was easily observed by controlling the concentrations in the mixed solutions. As a consequence, diverse changes in the absorption spectra were observed involving the formation of the cation radical and dication of the dimer compound, tetramethylbenzidine (TMB), depending on the concentration of DMABr. When only DMABr . (+) was formed, the reaction was assigned to the simple formation of TMB2+. This reaction was first order in DMABr . (+), and the rate constant was determined to be 0.20 s(-1). In contrast, when a large excess of DMABr was also present, the rapid and quantitative formation of TMB . (+) was observed via the reaction expressed by DMABr . (+) + DMABr --> TMB . (+) + Br-2. The validity of the ETSF method for spectrochemical analysis of electrogenerated cation radicals is demonstrated by analyzing the present reactions.