The mechanism of the photooxidation of tetraalkylammonium salts of phenylthioacetic acid, C6H5-S-CH2-COO-N+R4, was investigated in detail in order to understand why they turned out to be efficient co-initiators of free-radical polymerizations. The photosensitizer was benzophenone (BP), and the alkyl substituents (R) were n-butyl, n-propyl, or ethyl. The techniques used were steady-state and nanosecond flash photolysis. It was shown that electron transfer from the sulfur atom to the benzophenone triplet state was the primary photochemical step followed by a decarboxylation reaction leading to CO2, the C6H5-S-CH2 center dot radical, and a [BP center dot-center dot center dot center dot N+R4] ion pair. The latter underwent a Hofmann elimination (unexpected in these mild experimental conditions) leading to alkene-1 and trialkyl amine. The quantum yields of all the observed transients and the stable products were determined. The mechanism of primary and secondary photochemical reactions was quantitatively described, and it was shown to be similar for all of the alkyl derivatives used. The results of photo chemical studies were compared with respect to the polymerization studies where the systems consisting of benzophenone and tetraalkylammonium salts of phenylthioacetic acid (and phenylthioacetic acid for comparison) were used as photoinitiating couples for free-radical polymerizations using 2-ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate as the monomer. A linear correlation was found for the polymerization rates with respect to the square root of the CO2 quantum yields. This justified the hypothesis that the C6H5SCH2 center dot radical is the initiator of the free-radical polymerizations for the BP/C6H5-S-CH2-COO-N+R4 photoredox pairs studied in this work.