The chemistry of nitrated alkoxy radicals, and its impact on RO 2 measurements using the laser induced fluorescence (LIF) technique, is examined by a combined theoretical and experimental study. Quantum chemical and theoretical kinetic calculations show that the decomposition of β-nitrate-alkoxy radicals is much slower than β-OH-substituted alkoxy radicals, and that the spontaneous fragmentation of the α-nitrate-alkyl radical product to a carbonyl product + NO 2 prevents other β-substituents from efficiently reducing the energy barrier. The systematic series of calculations is summarized as an update to the structure-activity relationship (SAR) by Vereecken and Peeters (2009), and shows increasing decomposition rates with higher degrees of substitution, as in the series ethene to 2,3-dimethyl-butene, and dominant H-migration for sufficiently large alkoxy radicals such as those formed from 1-pentene or longer alkenes. The slow decomposition allows other reactions to become competitive, including epoxidation in unsaturated nitrate-alkoxy radicals; the decomposition SAR is likewise updated for β-epoxy substituents. A set of experiments investigating the NO 3 -initiated oxidation of ethene, propene, cis -2-butene, 2,3-dimethyl-butene, 1-pentene, and trans -2-hexene, were performed in the atmospheric simulation chamber SAPHIR with measurements of HO 2 and RO 2 radicals performed with a LIF instrument. Comparisons between modelled and measured HO 2 radicals in all experiments, performed in excess of carbon monoxide to avoid OH radical chemistry, suggest that the reaction of HO 2 with β-nitrate alkylperoxy radicals has a channel forming OH and an alkoxy radical in yields of 15-65%, compatible with earlier literature data on nitrated isoprene and α-pinene radicals. Model concentrations of RO 2 radicals when including the results of the theoretical calculations described here, agreed within 10% with the measured RO 2 radicals for all species investigated when the alkene oxidation is dominated by NO 3 radicals. The formation of NO 2 in the decomposition of β-nitrate alkoxy radicals prevents detection of the parent RO 2 radical in a LIF instrument, as it relies on formation of HO 2 . The implications for measurements of RO 2 in ambient and experimental conditions, such as for the NO 3 -dominated chemistry during nighttime, is discussed. The current results appear in disagreement with an earlier indirect experimental study by Yeh et al. on pentadecene. The chemistry of nitrated alkoxy radicals, and its impact on RO 2 measurements using the laser induced fluorescence (LIF) technique, is examined by a combined theoretical and experimental study.