To make clear the differences in kinetic behavior between cation radical and dication, detailed analyses were carried out on the reactions of electrogenerated 2,3,7,8-tetramethoxythianthrene cation radical (TMT+.) and dication (TMT2+) with nucleophiles (methanol and pyridine) by using a pulse-electrolysis stopped-flow method. Compared with the reactions of TMT+., which proceed via multi-step reactions including the electron transfer of another TMT+. to generate TMT before the rate determining step (rds), in the reactions of TMT2+ it was found that the rds is the first or the second step of the reactions; i.e. the unstable cationic intermediate seems to be formed directly and not via electron transfer. Besides the additional charge in the dication state, the mechanisms in which TMT2+ is easily accessible to the rds also suggest the high reactivity of TMT 2+ in comparison with TMT+.. The differences in the reaction mechanisms arising from the type of nucleophile were also discussed.