The thermolysis mechanism of di-tert-butyl and di-tert-amyl peroxide was investigated in a variety of solvents employing electron spin resonance in collaboration with the spin-trapping technique. The tert-amyloxyl radical was trapped for the first time during the thermolysis of di-tert-amyl peroxide. The relative ease of -scission of the tert-amyloxyl radical to give propan-2-one and butan-2-one was established as 32:1 at 382 K. In addition, competition reactions indicate that radicals derived from di-tert-butyl peroxide are ca. 5.5 times more reactive than those derived from di-tert-amyl peroxide towards hydrogen atom abstraction from toluene at 382 K.