Abstract—Reaction kinetics of radicals produced by the nanosecond laser flash photolysis of 2,2′‐dithiobis(pyridineN‐oxide) and related compounds have been studied. The transient absorption band at 360 nm was attributed to the radical in which the unpaired electron mainly localizes on the S atom (N‐oxypyridyl‐2‐thio radical). The reactivities of the radical for conjugated dienes are lower than those of the pyridyl‐2‐thio radical, suggesting that a considerable unpaired electron density on the S atom delocalizes onto theN‐oxypyridine moiety. The addition reaction rate of the radical to the conjugating diene was accelerated with hydrogen‐bonding solvents and with addition of the cation, which may stabilize the N+‐O‐canonical structure, increasing the unpaired electron density on the S atom. By the photolysis ofN‐hydroxypyridine‐2‐thione, the N‐O bond was predominantly dissociated producing a pyr‐idyl‐2‐thio radical. By the photolysis of its anion, photoejection took place followed by the N‐O