The formation of diphenyl nitroxide in diphenylamine inhibited, α,α′-azo-bis-isobutyronitrile initiated, autoxidations at 65° has been studied by electron spin resonance. Diphenylamine is oxidized to a diphenylamino radical which is then converted to the nitroxide by an oxygen atom transfer from a peroxy radical. The initial rates of conversion of diphenylamine to diphenyl nitroxide and the maximum nitroxide concentrations attained are generally greater for oxidations with tertiary peroxy radicals than for oxidations with primary or secondary peroxy radicals. The lower efficiency of nitroxide formation by primary and secondary peroxy radicals is attributed to a cage disproportionation between alkoxy radical and nitroxide which leads to the formation of a carbonyl compound and diphenyl hydroxylamine. This reaction cannot occur with tertiary radicals. The rate of formation of diphenyl nitroxide is greater for tertiary peroxy radicals which give stable tertiary alkoxy radicals. Nitroxide formation is inhibited by secondary, but not by tertiary, hydroperoxides.