A series of new bis(acyl)phosphane oxide (BAPO) photoinitiators has been synthesized and tested with respect to their efficiency 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 the most rapidly with styrene and the most slowly with butyl acrylate. Low fluorescence quantum yields of 0.1–0.3 % reveal that the studied BAPOs undergo efficient intersystem crossing, followed by α-cleavage. The heat profiles of selected photopolymerizations have been observed using a high-resolution infrared camera. Thermal-imaging experiments show substantial monomer-dependent exothermicity. All BAPO derivatives can additionally act as electron acceptors, as indicated by cyclic voltammetry and EPR spectroscopy.