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The initiation efficiency of acylphosphine-oxides has been evaluated in different types of UV-curable resins. Real-time infrared spectroscopy was used to monitor in situ the polymerization process in thin coatings exposed to UV-radiation. When compared to typical radical photoinitiators, acylphosphine oxides proved to be the most efficient ones with respect to both the polymerization rate and the cure extent, mainly because of their fast photolysis. Frontal polymerization proceeds readily with such photobleachable initiators, thus allowing thick specimens to be deep-through cured by simple exposure to sunlight. Because they absorb in the region of 350–400nm, acylphosphine oxides proved particularly well suited to induce the photopolymerization of both pigmented systems and protective coatings used in exterior applications, which contain UV-absorber-type light stabilizers. Superior performance was also …