Abstract. Peroxy radical reactions (RO 2 + RO 2 ) from the NO 3 -initiated oxidation of isoprene are studied with both gas chromatography and a chemical ionization mass spectrometry technique that allows for more specific speciation of products than in previous studies of this system. We find high nitrate yields (∼ 80 %), consistent with other studies. We further see evidence of significant hydroxyl radical (OH) formation in this system, which we propose comes from RO 2 + HO 2 reactions with a yield of ∼ 38-58 %. An additional OH source is the second generation oxidation of the nitrooxyhydroperoxide, which produces OH and a dinitrooxyepoxide with a yield of ∼ 35 %. The branching ratio of the radical propagating, carbonyl-and alcohol-forming, and organic peroxide-forming channels of the RO 2 + RO 2 reaction are found to be ∼ 18-38 %, ∼ 59-77 %, and ∼ 3-4 %, respectively. HO 2 formation in this system is lower than has been previously assumed. Addition of RO 2 to isoprene is suggested as a possible route to the formation of several isoprene C 10 -organic peroxide compounds (ROOR). The nitrooxy, allylic, and C 5 peroxy radicals present in this system exhibit different behavior than the limited suite of peroxy radicals that have been studied to date.