The reactions of 3-substitute d-diphenylamine cation radicals in acetonitrile were studied using an electron transfer stopped-flow method. In the reactions of the 3-chloro-diphenylamine cation radicals (mCl-DPA(.+)), the main reaction route was the formation of the benzidine dimer, which was similar to the case of the diphenylamine cation radical (DPA(.+)). Although the reaction of DPA(.+) proceeded via the cation radical - cation radical coupling as verified from the rate law of -d[DPA(.+)]/dt = k [DPA(.+)](2), the present kinetic analysis has revealed that the decay rate of mCl-DPA(.+) was dependent on the concentration of the neutral molecules, i.e., the rate law was expressed as -d[mCl-DPA(.+)]/dt = k [mCl-DPA(.+)]1 [mCl-DPA]. In contrast, the reaction of the 3-methoxy-diphenylamine cation radical (mMeO-DPA(.+)) was too fast to be observed using the stopped-flow method, which is quite in contrast to the 4-methoxy-diphenylamine cation radical (pMeO-DPA(.+)) which was very stable in acetonitrile. In the case of mMeO-DPA(.+), the cyclization reaction was confirmed to proceed soon after the generation of mMe-ODPA(.+), which is similar to the case of the 3-methyl-diphenylamine cation radical (mMe-DPA(.+)). Thus, it was found that the substituent on the 3-position changed the reaction pathways of DPA(.+) significantly, as well as their reactivity.