The decay reaction of N,N-dimethyl-p-toluidine (DMT) cation radical (DMT.+) in acetonitrile (AN) was analyzed using an electron-transfer stopped-flow (ETSF) method. In the ETSF method, DMT.+ is generated by mixing AN solutions of DMT and tris(p-bromophenyl)amine cation radical (TBPA(.+)). When DMT.+ was generated quantitatively without DMT via 1:1 mixing of DMT and TBPA(.+), it was found that DMT.+ was fairly stable in AN. On the other hand, when DMT remained with DMT.+ under the control of the mixing ratio of DMT/TBPA(.+) (> 1), the neutral DMT was found to promote the decay reaction of DMT.+. From the determined rate law, -d[DMT.+]/dt = k [DMT.+] [DMT] (k = 6.5 x 10(2) M-1 s(-1)), the initial acid-base reaction between DMT.+ and DMT was clarified to be the rate determining step. The acid-base interaction was also confirmed by observing the decay reaction of DMT.+ in the presence of pyridine derivatives. The identical rate law, which indicates the rate-determining acid-base interaction, was obtained for eight pyridine derivatives examined, though the final product was different from the case of DMT. The ETSF method has permitted the straightforward analysis and given a definite kinetic conclusion concerning the acid-base reaction between DMT.+ and DMT.