Initiation by tert-butyl peroxypivalate (TBPP), tert-amyl peroxypivalate (TAPP), 1,1,3,3-tetramethylbutyl peroxypivalate (TMBPP), or 1,1,2,2-tetramethylpropyl peroxypivalate (TMPPP) of radical polymerization of methyl methacrylate in toluene solution at 90°C was studied via polymer end-group analysis using electrospray ionization mass spectrometry (ESI-MS). Conclusive peak assignments allowed for measuring the type and concentration of the fragments that actually initiate macromolecular growth after thermal decomposition of these peroxypivalates. It was found that the pivaloyloxy radical moiety undergoes instantaneous decarboxylation to yield an initiating tert-butyl radical. The alkoxy radical moiety, on the other hand, may generate, via b-scission reaction, different types of carbon-centered radicals (together with a ketone) or may undergo a 1,5-H-shift reaction, by which reaction an oxygen-centered radical is transformed into a carbon-centered hydroxy radical. This hydrogen shift reaction was found in case of TMBPP. Surprisingly, no evidence for initiating alkoxy radicals could be found, not even in case of initiation by TBPP, where the intermediate tert-butoxy radical undergoes a rapid chain-transfer reaction with the solvent toluene.