The initiation mechanisms of 1,1,2-trimethylpropyl peroxypivalate 1a and 1,1,2,2-tetramethylpropyl peroxypivalate 1b in the radical polymerization of styrene and methyl methacrylate (MMA) have been studied using the nitroxide trapping technique. Thermolysis of 1 generated t-butyl and the corresponding t-alkoxyl radicals, i.e. 1,1,2-trimethylpropoxyl 2a and 1,1,2,2-tetramethylpropoxyl radicals 2b. Both t-alkoxyl radicals underwent very fast unimolecular processes (β-scission) essentially to the exclusion of intermolecular processes (addition and H-abstraction), in contrast to other t-alkoxyl radicals such as t-butoxyl radicals. The extent of β-scission of 2a and 2b to form alkyl radicals R· were 97.6 and 99.7% in styrene and 98.4 and 99.7% in MMA, respectively. Alkyl radicals formed in the reaction then underwent selective tail addition to monomers or were trapped by the nitroxide. From the relative yields of products arising from the competitive addition/trapping reactions of alkyl radicals, the absolute rate constants for the addition of isopropyl radicals to the two monomers at 60°C are estimated to be 4.7×105 l mol−1 s−1 to styrene and 1.3×106 l mol−1 s−1 to MMA, respectively.