Amine- and phosphine-ligand-containing borane complexes (BoC) are proposed as new, highly efficient coinitiators for acrylate photopolymerization. In the presence of benzophenone (BP), they are highly reactive and lead, in many cases, to similar or even better polymerization rates than those obtained with a reference amine coinitiator (ethyl dimethylaminobenzoate, EDB). Interestingly, the photopolymerization reaction under aerated conditions is almost unaffected. Laser flash photolysis, ESR, and DFT calculations were also used. The direct detection of the boryl radicals, their transient absorption spectra, and their interaction with acrylates and oxygen as well as the peroxyl radicals/BoC and (BP)-B-3/BoC interactions have been studied and allow us to explain the photoinitiation step mechanism under air.