Experiments at atmospherically relevant conditions were performed in the simulation chamber SAPHIR, investigating the reaction of isoprene with NO3 and its subsequent oxidation. Due to the production of NO3 from the reaction of NO2 with O3 as well as the formation of OH in subsequent reactions, the reactions of isoprene with O3 and OH were estimated to contribute up to 15% of the total isoprene consumption each in these experiments. The ratio of RO2 to HO2 concentrations was varied by changing the reactant concentrations, which modifies the product distribution from bimolecular reactions of the nitrated RO2. The reaction with HO2 or NO3 was found to be the main bimolecular loss process for the RO2 radicals under all conditions examined.Yields of the first-generation isoprene oxygenated nitrates as well as the sum of methyl vinyl ketone (MVK) and methacrolein (MACR) were determined by high resolution proton mass spectrometry using the Vocus PTR-TOF. The experimental time series of these products are compared to model calculations based on the MCM v3.3.1,1 the isoprene mechanism as published by Wennberg et al.2 and the newly developed FZJ-NO3-isoprene mechanism,3 which incorporates theory-based rate coefficients for a wide range of reactions.Among other changes, the FZJ-NO3-isoprene mechanism contains a novel fast oxidation route through the epoxidation of alkoxy radicals, originating from the formation of nitrated peroxy radicals. This inhibits the formation of MVK and MACR from the NO3-initiated oxidation of isoprene to practically zero, which agrees with the observations from chamber experiments. In addition, the FZJ-NO3-isoprene mechanism increases the level of agreement for the main first-generation oxygenated nitrates. 1 M. E. Jenkin, J. C. Young and A. R. Rickard, The MCM v3.3.1 degradation scheme for isoprene, Atmospheric Chem. Phys., 2015, 15, 11433-11459.2 P. O. Wennberg at al., Gas-Phase Reactions of Isoprene and Its Major Oxidation Products, Chem. Rev., 2018, 118, 3337-3390. 3 L. Vereecken et al., Theoretical and experimental study of peroxy and alkoxy radicals in the NO3-initiated oxidation of isoprene, Phys. Chem. Chem. Phys., submitted.