The thermolysis of di-tert-butyl and di-t-amyl peroxides in n-decane and with poly(methyl methacrylate) and poly(ethylene glycol) in chlorobenzene has been studied in the presence of tri-tert-butylnitrosobenzene as a spin trap. The major reaction pathways of the resulting t-alkoxyl radicals were β-scission and hydrogen atom abstraction. Thermolysis of the same peroxides in the presence of methyl methacrylate and methyl acrylate led to the trapping of growing polymer chains alongside unreacted alkyl radicals (again formed via β-scission of the first generation t-alkoxyl radicals). In some of the spectra, hyperfine coupling to γ-protons was observed. The hydrogen abstracting ability of radicals present during thermolysis of both peroxides has been compared in the presence of MA.