Although the spectroscopic detection of electrochemically generated pyrene cation radical (PY.+) is generally difficult at room temperature in acetonitrile (AN), we could successfully observe the time changes in the absorption spectra of short-lived PY.+ using an electron transfer stopped-flow (ETSF) method by adopting the tris(2,4-dibromophenyl)amine cation radical (TDBPA(.+)) as a reaction initiator. The decrease of PY + was observed to be complete within 30 ms after generating PY.+ via the mixing of the AN solutions of PY and TDBPA(.+). This decay reaction of PY.+ can be attributed to the reaction with a trace amount of water in AN. Thus, the reactions of 1-substituted pyrene cation radicals (XPY.+) with water were systematically analyzed using the ETSF method by adding a constant amount of water. Consequently, the reactions were confirmed to be second-order in the cation radicals in all the cases. Among the derivatives, the higher stability of 1-aminopyrene(.+) was confirmed on the basis of the slow reaction rates, and the order of the reactivity toward water was clarified to be pyrene(.+)>1-bromopyrene(.+)>1-methylpyrene(.+)>1-aminopyrene(.+). The present results of XPY.+ were compared with our previous results on the reactions of 9-substituted anthracene derivative cation radicals. (C) 2004 Elsevier B.V. All rights reserved.