The reaction intermediates produced by the nanosecond-laser-flash photolysis of N-hydroxypyridine-4-thione (4-HOPyT) have been studied in aqueous and nonaqueous solutions. The pyridyl-4-thiyl radical (4-PyS˙ at 420 nm) and ˙OH are formed by the homolytic N–OH bond cleavage of 4-HOPyT. The 720-nm band was assigned to the pyridyl-N-oxyl radical (4-SPyO˙) generated by the photodissociation of the O–H bond and by H abstraction from 4-HOPyT through ˙OH. The triplet state of 4-HOPyT [³(4-HOPyT)*], confirmed by energy transfer with triplet quenchers, is also produced concomitantly with the radical species. Diffusion controlled electron-transfer reactions occur between ³(4-HOPyT)* and donors (or acceptors), which may be caused by the relatively high triplet-state energy (E_T1 = 60.1 kcal mol^–1) of ³(4-HOPyT)*. In aqueous alkaline solution, the solvated electron (e_aq^–) and the one-electron semioxidized radical of 4-HOPyT (4-SPyO˙) are produced by photoionization of the anionic form of 4-HOPyT. Thus, it is possible to control the generation of ˙OH and e_aq^– by changing the pH of the solution.