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 350-400 nm, 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 achieved with these photoinitiators in the photocrosslinking of functionalized polymers, such as acrylated polyisoprene and polybutadiene with pendent vinyl groups. The addition of small amounts of a trifunctional thiol was shown to drastically speed up the crosslinking polymerization, insolubilization being achieved within a 0.3 s UV-exposure, even for initiator concentrations as low as 0.1 wt%.