Photochemical reactions of 6H-purine-6-thione (PuT) via the excited triplet state [(3)(PuT)*] have been studied by means of laser flash photolysis in organic solvents. Transient absorption bands at 475 and 690 nm were assigned to (3)(PuT)*. Intersystem quantum yield and the lowest triplet energy of (3)(PuT)* were evaluated to be 0.99 and 63 kcal/mol, respectively. The self-quenching rate constant is quite large (2.3 x 10(9) M-1 s(-1) in THF). In photoinduced electron transfer, (3)(PuT)* acts as electron acceptor for tetramethylbenzidine, while (3)(PuT)* acts as electron donor for p-dinitrobenzene. Rate constants for H-atom abstraction (k(hT)) Of (3)(PUT)* from benzenethiols, tocopherol, and 1,4-cyclohexadiene are on the order of 10(8) M-1 s(-1). From the Hammett plots of k(hT) for substituted benzenethiols, a negative rho value indicates that (3)(PuT)* has electrophilic character. In the addition reaction of (3)(PuT)* toward various alkenes, the electrophilic character of (3)(PuT)* was also confirmed. By steady-light photolysis of PuT, purine was produced via (3)(PuT)* after H-atom abstraction. On combination of these results, the character of the lowest (3)(PuT)* was presumed.