Abstract In order to clarify the primary photochemical process of 4–nitropyridine N–oxide in solution, kinetic flash spectrophotometric studies were carried out under various conditions. The flash excitation of 4–nitropyridine N–oxide in alcoholic solution resulted in three transient species in the spectral region between 360 nm and 580 nm; one of them was a weak absorption at about 530 nm (transient species A and A′), and the other, a stronger one around 380 nm (transient species B). The experiments on the decay kinetics of these transients revealed that the photochemical reaction of 4–nitropyridine N–oxide in an alcoholic solution proceeded through these intermediate species originating from the excited singlet state; Process I (the photochemical formation of 4-hydroxypyridine N-oxide nitrate) involved the bimolecular interaction between an intermediate A′ and unexcited molecule, while Process II (the photochemical formation of 4-hydroxylaminopyridine N-oxide) was initiated by a hydrogen-atom abstraction from a solvent molecule by an intermediate B which was produced from another intermediate A. The two intermediates A and A′ were reasonably assumed to be formed concurrently from the excited singlet state of 4-nitropyridine N-oxide.