The effects of a variety of excited state quenchers on the four major photochemical reaction modes of n-butyraldehyde in the vapour phase and in isooctane have been studied by monitoring primary reactions through the determination of characteristic product quantum yields. Singlet state and triplet state contributions to these photochemical processes were obtained from the quenching plots. Thus it was shown that in the vapour phase 50% of type (II) decomposition occurred from the excited singlet state whereas in isooctane only 20% of type (II) decomposition followed this route. Triplet state lifetimes of 3 × 10-7 s in the vapour phase and 5 × 10-7 s in solution were derived from a kinetic analysis of the quenching data. It was shown that singlet energy transfer to cis-piperylene occurred with high efficiency, the rate constants being close to the collision-controlled limit in the vapour phase and to the diffusion-controlled limit in solution.