in the initiation step of radical photopolymerizations. The transient absorption spectra of the phosphanoyl radicals obtained upon laser‐flash photolysis reveal maxima at ca. 450–460 nm. Rate constants for the addition of these radicals to the double bonds of butyl acrylate, methyl methacrylate, 1‐vinyl‐2‐pyrrolidone, and styrene have been determined. All phosphanoyl radicals have been found to react