Reversible addition fragmentation chain transfer (RAFT) mediated methyl acrylate and butyl acrylate polymerizations initiated via Co-60 gamma -irradiation were investigated using electrospray ionization mass spectrometry (ESI-XIS). The polymer end-groupings were mapped using both a hybrid quadrupole-time-of-flight instrument and a quadrupole ion trap mass spectrometer. Three potential initiation processes were identified during the cumyl phenyl dithioacetate (CPDA) mediated acrylate polymerizations. CPDA dissociates into a cumyl radical and a phenyl dithioester radical (Z-group). A set of polymer product ions indicates that a small, yet significant, fraction of the gamma-generated pheryl dithioacetate radical contributes to the initiation process. Furthermore, the phenyl dithioacetate group undergoes cleavage to yield thiol end groups that react to form disulfide linkages. Additionally, polymer product ions indicative of a small amount of water radiolysis were observed, and evidence that the butyl acrylate monomer interacts with gamma-radiation to yield an initiating species is gained.