Significance Unimolecular hydrogen shift reactions to peroxy radicals have been shown to be important in the atmospheric photooxidation of isoprene and α-pinene. These studies also report the efficient generation of highly oxidized organic molecules known to contribute to particle formation and growth. Here, we quantify the rate of this oxidation pathway for peroxy radicals produced in the oxidation of n -hexane under conditions relevant to the atmosphere. The results suggest that autoxidation pathways are competitive against bimolecular reactions for a broad range of substrates, including many that result from urban emissions. The formation of organic hydroperoxides from atmospheric autoxidation has unknown implications for air quality.