The mechanism of the benzopbenone-sensitized photooxidation of phosphonium salts of (phenylthio)acetic acid was studied as a means for understanding how these salts function as coinitiators in the free-radical photopolymerization of vinyl monomers. Both steady-state and nanosecond flash photolytic methods were used to determine, in a quantitative manner, the mechanism of primary and secondary photoreactions for three quaternary phosphonium salts containing butyl and/or phenyl groups, i.e., P+(C4H9)(4), P+(C4H9)(C6H5)(3), and P+(C6H5)(4). It was found that the initial polymerization rates were the same for all three phoshonium salts of (phenylthio)acetic acid and were equal to those found previously for tetralkyl ammonium salts. The polymerization rates were more than twice the rates found for direct initiation by benzophenone and by the benzophenone-(phenylthio)acetic acid initiating system. These results correlate well with the large quantum yields of (CH2SC6H5)-C-center dot radicals (the main initiating radicals) found in the complementary photochemical investigation. It was found that a detailed knowledge of the photochemical reactions in the photoinitiating systems was critical to understand the kinetics of polymerization. (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 8013-8022, 2008