Photolytically generated radicals (at a wavelength of 351 nm) derived from the acetophenone-type photoinitiators benzoin (2-hydroxy-1,2-diphenylethanone) and mesitil (1,2-dimesitylethane-1,2-dione) (specifically the benzoyl and mesitoyl radicals) are quantified in their ability to serve as initiating species in methyl methacrylate (MMA) bulk free radical polymerizations at 5 °C. The polymerizations are initiated by the pulsing action of a high-frequency excimer laser (100 Hz) operated at the 351 nm XeF line. The pulsing action of the laser serves to limit the molecular weight of the generated polymer to allow its analysis via electrospray ionization mass spectrometry (ESI-MS) employing a quadrupole ion trap analyzer. MMA-derived propagating radicals terminate to a large proportion via disproportionation. These disproportionation products can be unambiguously detected via ESI-MS. The resulting disproportionation signals canswithin each repeat unit and over the entire mass spectrumsbe quantitatively evaluated. The ratio of benzoyl and mesitoyl end groups is demonstrated to be independent of the polymer chain length, and no mass bias is observed in the ESI-MS spectra. Benzoin and mesitil are employed in variable ratios in the reaction mixture as a cocktail. A plot of the ratio of benzoyl to mesitoyl end groups vs the ratio of both initiators in the reaction mixture indicates that the benzoyl radical is 8.6 times more likely to initiate the polymerization process than the mesitoyl fragment.