Acylphosphine oxide photoinitiators, such as phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO) and diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO), are widely used as sensitisers in UV polymerisation applications due to their favourable properties for UV lithography. However, the underlying photophysical and photochemical mechanisms that govern their performance remain poorly understood. In this study, we systematically investigate these compounds using steady-state and time-resolved fluorescence, as well as electronic and infrared transient absorption spectroscopic techniques. The results show that the lowest electronic excitation of both BAPO and TPO leads to a rapid intersystem crossing (ISC) into the hot triplet manifold-which rapidly generates radicals, further undergoing relaxation and leading to a slower rate of radical formation from the cold triplet manifold. ISC is faster in TPO compared to BAPO, which exhibits more complex dynamics. In addition, time-resolved infrared data reveal that both sensitisers generate radicals (by α-cleavage) on a time scale similar to that of ISC, suggesting that the hot triplet state is very short-lived. Finally, nanosecond-to-microsecond electronic TA spectroscopy revealed multi-stage relaxation of the radicals with the formation of TPO photoproducts remaining detectable on microsecond time scales.