The type and number of end groups of poly(methyl methacrylates) from free-radical polymerization with six diacyl peroxides, R–(CO)O–O(CO)–R, acting as initiators have been analyzed via electrospray ionization mass spectrometry using an ion trap and additionally Fourier transform ion cyclotron resonance for mass detection. The polymerizations were carried out in benzene solution at high initiator concentration to yield low molecular weight polymer. With R being an alkyl group, only R moieties are observed as end groups. For each oligomer size, molecules with one or two such end groups are formed, depending on whether termination occurs via disproportionation or combination. With R being an aryl type, as in di-benzoyl and di-naphthoyl peroxides, both R and R–(CO)O moieties are detected as polymeric end groups. Because of aromatic delocalization, fractions of the arylic R–(CO)O. radicals are sufficiently long living at 95 °C to add to a monomer molecule prior to undergo decarboxylation.