The dimerization reaction of N,N-dimethyl-p-toluidine (DMT) cation radical (DMT + ) in acetonitrile (AN) was analyzed using an electron transfer stopped-flow method. In this method, DMT + was generated by mixing AN solutions of DMT and tris(p-bromophenyl)amine cation radical (TBPA + ). When DMT + was generated quantitatively without DMT, it was found that DMT + is fairly stable in AN. On the other hand, by controlling the mixing ratio of DMT/TBPA + , the effect of neutral DMT on the dimerization of DMT + was analyzed. Consequently, it was found that the neutral DMT accelerated the decrease of DMT + significantly. From the determined rate law, -d[DMT + ]/dt=k[DMT + ] [DMT] (k=6.5x10 2 M - 1 s - 1 ), the initial reaction of DMT + with DMT was clarified to be the rate determining step. Thus, the acid-base interaction between the cation radical and the neutral molecule was revealed in the present case of DMT. This was also confirmed by observing the reaction between DMT + and pyridine.