Photoinitiators play a key role in UV-curable systems by generating the reactive species, free radicals, ions or molecule which initiate the polymerisation of the multifunctional monomers and oligomers. This research involves the study of the hydrolytic and photolytic stabilities of two important new photoinitiator bis (2,4,6-trimethylbenzoyl)phenylphosphine oxide (BAPO) and (2,4,6-trimethylbenzoyl)diphenyl-phosphine oxide (TPO). BAPO is a new member in the group of well-known α-cleavable photoinitiators among which monoacylphosphine oxide (MAPO) is most recognised. These α–cleavable photoinitiators are characterised by low volatility and high solubility in acrylate monomers and they are very widely applied as curing agents for white and pigmented coatings including furniture coatings. Novel analytical methods were developed in order to characterise these compounds and to understand their modes of action and breakdown. The stability of BAPO in different solvents, such as acetonitrile and a cyanoacrylate ester, was studied and the effect of storage of BAPO in light and dark conditions was evaluated. The effects of stabilisers (ferrocene, cumene hydroperoxide (CHP) and hydroquinone (HQ)), acids (methyl sulphonic acid (MSA) and boron trifluoride dihydrate (BF3∙2H2O)) and water present in the solvent were also investigated. These studies were monitored over time using LC-UV and CE-UV methods. Mass spectrometry was employed for the first time to investigate the pathways and photodegradation products for both of the photoinitiators BAPO and TPO. NMR spectroscopy and computational studies were used as complementary experimental and theoretical techniques respectively for the elucidation of the photochemistry of these products. A formulation study using the photoinitiator and ethyl cyanoacrylate monomer to ascertain the polymerisation products, and possible cure mechanism with the aid of mass spectrometry was also carried out.