The photochemistry of 4-azidopyridine-1-oxide was investigated by room-temperature laser flash photolysis, glass matrix photolysis with ultraviolet/visible spectroscopy and product analysis nuclear magnetic resonance spectroscopy, and electron paramagnetic resonance spectroscopy. It was determined that at room temperature, intersystem crossing to the triplet state is the dominant process, yielding 4,4' - axobbis(pyridi-ne- l -oxide) as the only photoproduct.After photolysis in a 3-methylpentane glass matrix at 77K, strong NMR and HPLC evidence suggested that hydrogen abstraction from the solvent is the dominant process, yielding 4-(1' -ethyl-1' - nethylpmpyl)amdopyridine-1-oxide. This result was consistent with the results of a comparative study of the same process with the isomeric 3-azidopyridine-1-oxide. Photolysis in 2-methyltetrahydrofuran glass gave a complicated product mixture that was not investigated further.A kinetic investigation of the 1,3-dipolar cycloaddition reaction of 4-azidopyridine-1-oxide with methyl acrylate in acetonitrile yielded a second-order overall rate coefficient of 1.00 x 10'' M-1s-1 at 323 K. From this study, it was also concluded that the first cycloaddition step in the mechanism of the reaction is rate-determining.